It's not so much important the time elapsed but possibly the storage conditions: heat, humidity, dust, and who knows, vibrations.
If you can inspect the camera before purchase, check fungus on lens, dust, clean viewfinder, working rangefinder if the camera has got one, make some shots without film, check if the shutter noise has something anomalous, check battery contacts, put battery, etc.
If you cannot inspect the camera before purchase, look for seller attitude. If he can describe the camera in detail, if he is ready to show it to you personally (regardless of whether you really have the intention to go and see it), if he's ready to let you take a roll with it, and is prepared to take it back in case of problems, you might probably trust him.
If he says he doesn't know anything about camera, he has no idea about its state, he gives no guarantee, then I'd rather make a very low offer (means less than $50) because you have to factor in the price of some maintenance for sure. Don't be lured by good-looking aspect in any case. Test the seller.
Sometimes the seller really doesn't know about cameras, he knows the camera doesn't work, dishonestly plays the inexpert guy, and actually ignores that the problem can be solved with $40 or so. Surprises are often not that expensive to repair. And if the camera is not repairable, you have plenty of spare pieces in almost new condition. You can also sell it as camera for parts and recover your expense.