There is a huge range depending on how it's jetted. Most bikes from that era are now rejetted for different exhausts and intakes so it will depend on how precisely the cruising part of the throttle is jetted.
I would say anywhere from 40 to 60 MPG is typical. On my 1981 Gpz550's crsuising 60 to 70 mph is in the 1/16 to 1/8 throttle range so when I rejet, I really focus on that area as the limit for lean operation. everywhere else on the throttle will have to be richer. If I live with some extra choke duration in the cold, I can jet it so that cruising will easily get over 60 MPG. That is with the bike still running perfectly fine etc. with the airbox. The airbox makes the airflow more consistent so you can gleam it leaner without any hesitation. Pods need to be a tiny bit richer to make up for things like crosswinds etc.
I'm not sure what a real-world MPG value would be on a stock bike. Remember all magazine tests from back in the day probably used real gas with little to no alcohol. Ethanol gives slightly less miles per gallon.
(These are U.S. gallons, and running U.S. 93 Octane, usually with 5 to 10 % ethanol.)