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It takes quite a while for the gasoline to "varnish".  The fuel might go bad in a few weeks (not varnish), but the motorcycle should still start & run if it isn't too old.  Before trying to start, be sure to prime the carburetors beforehand using the petcock selector, in case the fuel level in the carb bowls is low for whatever reason, such as evaporation.  The engine may be a little hard starting but it should start; it may run poorly at first but should improve as the fuel from the tank refills the carb bowls.  All of this has been my experience, but my motorcycle is in a good state of tune & normally starts the second I touch the starter button. so old fuel doesn't cause a problem.  If your motorcycle is already hard starting, then dank fuel could be troublesome.

For storage I would drain all of the old fuel from the carburetors & fuel tank, then refill with fresh fuel.  I would to prime the carburetors to fill them with fuel.  You could also add a fuel stabilizer, but I don't use that stuff as we can have good riding weather year round.

Fuel freshness is dependent upon time, temperature & quantity.  Fresh fuel burns better than older fuel.
Higher temps cause fuel to go bad sooner than cooler temps. in summer you have to be aware of that. Winter time isn't as big of a problem.
Large quantities of fuel last longer than smaller amounts.  so the gasoline in the carburetors can go bad before the fuel in the tank.  Sometimes I'll drain the carbs, then prime to refill. then burn the old carb fuel in the lawnmower.

 

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I fill the bikes with non ethanol gas in the winter and start them at least once per month when not riding regularly. I've been bitten by the glowing, green gas so I am extremely careful with my vehicle fuel.  



 

Never seen that, what causes it?

 

No idea. This is from my Triumph GT6 after sitting for a few months. The same thing happened to my MX bike when it was parked for six months due to surgery. 

I always noticed that after a day or two it would start and idle perfect then after 4 or 5 days be a little less so and a week or two it would be a little rougher with a mild back fire or two till it cleaned out.

A month twice over the years have gummed em up. Was like a oven though in the shed so that adds to the problem I.m sure. I like Ness's idea of adding Stable for that reason. And when I drain the carbs for the winter I spray a little carb cleaner through the drain plug just to get most of the junk that may have ended up in the bottom there.

As long as we are tossing out opinions

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There is also a video from Jay Leno's Garage where he concludes that ethanol-free fuel is the only solution.

Since I've started using Stabil in the fuel, for both my bikes and garage gas can feeding my lawnmower etc, I've yet to have any fueling issues with my gasoline driven equipment.  Call me SOLD!  Prior to that my bike pilot jets used to plug up at about the four month mark using crappy California gasoline.  Very frustrating for someone with more than one bike, some of which sat for longer than should be the case.  As a bonus, Stabil helps protect the inside of the gas tank too and eliminates corrosion.  

I have seen several videos like this, claiming it is pretty easy to remove the ethanol.

Hi, i have a couple of 2 strokes that i restored five years ago or so, after talking to a stall holder at a auto jumble he persuaded me to try a fuel stabilising  tablets that you just drop into the tank and forget about, they were developed in the second world war for the spitfires and like to preserve the fuel. about two months ago i thought i should remove the carbs and give them a good cleaning out, i shouldnt of bothered as upon strip down they were as clean as when i put them together all those years ago !
As i can remember they were about ten pounds each and are supposed to last at least five years and make the fuel burn slightly cooler. will be buying a couple more for my newer projects as i was that impressed !

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I have seen several videos like this, claiming it is pretty easy to remove the ethanol.

Yes! it is that simple to remove the ethanol.
You just have to watch the octane drop too and compensate for that.
Whatever you do though, especially if running these bikes regularly is that it's going to be either expensive or inconvenient to feed them the correct fuel in the future.
 

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Since I've started using Stabil in the fuel, for both my bikes and garage gas can feeding my lawnmower etc, I've yet to have any fueling issues with my gasoline driven equipment.  Call me SOLD!  Prior to that my bike pilot jets used to plug up at about the four month mark using crappy California gasoline.  Very frustrating for someone with more than one bike, some of which sat for longer than should be the case.  As a bonus, Stabil helps protect the inside of the gas tank too and eliminates corrosion.  

  
I'm buggered if I know why no one else commented on this ED, Stabil is widely used  in Australia due to our 24/7, 365 use of boats , we have the highest recreational use of boats in the world , and Stabil stops the fuel from "going off" due to sitting in dormant fuel tanks. We don't use it much in bikes here as we ride year round, but If I had numerous bikes in my shop, and didn't ride them all the time,  they'd all have Stabil in them....  And I NEVER use ethanol fuel, we have a high humidity environment and ethanol fuel absorbs moisture...

On a related note...

I've started using the ethanol-free gasoline from the station down the road.  I added 2 gallons a month ago, and 1.5 gallons last week (to an existing tank of ethanol blend fuel).  This morning I went out on a ride and the carbs were spilling fuel out of the intake. 
I believe that the dilution/elimination of the ethanol from my fuel system caused the rubber on my float seals to shrink, allowing the carbs to overflow.  That's the only thing I can think of as it looked like fuel was spilling out of all 4 carb mouths and the fuel blend is the only thing that's changed since I rode it last.

wrote:

On a related note...

I've started using the ethanol-free gasoline from the station down the road.  I added 2 gallons a month ago, and 1.5 gallons last week (to an existing tank of ethanol blend fuel).  This morning I went out on a ride and the carbs were spilling fuel out of the intake. 
I believe that the dilution/elimination of the ethanol from my fuel system caused the rubber on my float seals to shrink, allowing the carbs to overflow.  That's the only thing I can think of as it looked like fuel was spilling out of all 4 carb mouths and the fuel blend is the only thing that's changed since I rode it last.

I don't think the change in fuel is the issue.  The rubber parts in the fuel system are nitrile and the effect on them won't differ much between pure gasoline vs. gasoline with ethanol.  

wrote:

wrote:

On a related note...

I've started using the ethanol-free gasoline from the station down the road.  I added 2 gallons a month ago, and 1.5 gallons last week (to an existing tank of ethanol blend fuel).  This morning I went out on a ride and the carbs were spilling fuel out of the intake. 
I believe that the dilution/elimination of the ethanol from my fuel system caused the rubber on my float seals to shrink, allowing the carbs to overflow.  That's the only thing I can think of as it looked like fuel was spilling out of all 4 carb mouths and the fuel blend is the only thing that's changed since I rode it last.

I don't think the change in fuel is the issue.  The rubber parts in the fuel system are nitrile and the effect on them won't differ much between pure gasoline vs. gasoline with ethanol.  
 

I had that thought as well, but it's the only thing that's changed since my last ride.  Obviously a tear down is in order.