Top-end replacement intervals can vary widely from rider to rider, depending on how hard you ride, how well you maintain your bike, engine size, etc. If you like to rev the snot out of your bike, the top-end will need replacing more frequently than a rider that rides more conservatively, if you neglect your air filter, don't run enough oil in your gas, etc...What I would reccomend is after you do your first rebuild and break it in, do a compression test, and write the result down. Repeat the test every few hours of riding, and once the engine has lost 10-15% of it's original reading, it is time for a top-end. In general, 20 hours is a long time on a piston in an 80 for someone that is racing and pushing the engine hard all the time, but someone that just casually trail-rides may get twice this or more. This is an area you don't want to push too long, because a piston failure from running it too long can become a very expensive repair job, costing several hundred dollars, instead of the 100 or so dollars replacing the top-end when it was due would have cost.