As with anything else, regular maintenance is the key to long life and, if done regularily, your genset should give many years of reliable service. How many hours is impossible to say, but regular use is much better than leaving it sit for weeks or months at a time, and occasional heavy use helps keep the engine and slip rings clean and running properly. Failures can occur, but with a well maintained unit, they are usually minor - carburetor adjustment/rebuild, fuel pump, and (rarely) an electronic control board.
My Onan 7000 has almost 500 hrs. on it now. We ran it continuously a few months ago for 58 hrs. while dry camping with no problems whatsoever. Our local Onan repair facility said that it shouldn't need a tuneup until about 750 hrs. We do change the oil and filter at least once per year or sooner if we have used it heavily.
NOT running a generator is more damaging than using it. We fire ours up for at least an hr. once per month during winter storage time.
Are you asking how long of a service life will it have, i.e. how long between major overhauls (not just tune ups)? I would estimate thousands of hours - at least 2000 and probably more. Longer for diesels.