if you bring me the tranny, i'll look at it and tell you what's screwed, no cost, since i'm not really doing any work.

i can tell you the first things to look for will be scores/wear marks on the end of the shift forks, and worn edges of the shift drum where the forks run in it. all the edges should be nice and crisp, and if it's worn, they'll be rounded off.

as far as "often"... usually the tranny will pop out of gear when it has alot of load on it, like when you're accelerating hard. if it starts doing that, then you have a problem.