Depends on the flash, I suppose. My only flash is a Sunpak that was probably about five years old when I got it from my uncle. That was twenty years ago. It's still going strong. Probably not the most heavily used flash in the world, but it is a flash that gets used, and those years certainly do add up.
In fact, that very flash is in my bag here at work, just in case. If I were buying a second hand flash, my greatest concern would be that it had been properly looked after, hadn't been dropped, etc. I wouldn't be concerned for the life of the thing, especially for only a couple of years, unless perhaps it had been used in a studio for thousands of shots every day the entire time.