I don't think I need to ride it more than a few hundred yards, I just need to check gears switching(200 yards?). Maybe 1 mile top. I can check engine of parked bike (no smoking/knocking). Check all lights if you want w/o riding.
If you buy water-cooled bike, you want to make sure, it doesn't overheat, you'll need 5 miles or so.
How far do you need to ride air-cooled bike to check? What exactly are you going to check riding a bike for 30 min.?

 

No funny noises. If it sounds wrong, it probably is. If it feels wrong, it probably is. If it looks neglected and abused, it probably is. Unless a bike has been sitting in a barn for 30 years, it should look and run as nice as possible, if it doesn't, find out why or walk away.