How long to run the engine before reading plugs?

How long does the engine need to run at specific throttle inputs before you can read the sparkplugs correctly?

Bonus question:
The Mikuni BS34's (KZ700-A1, KZ750-L3, etc.) mixture screws are adjusting fuel, not air right? In is leaner, out richer?

After warm up, 1/4, 1/2, 34/ throttle under load. Kill the engine, coast to a stop.
In is lean/out is rich.
Steve

But for how long to get enough colour on the plugs? I've only done it way back when and read that it takes longer with modern petrol?