Most serious "pattern" fliers put a lot of time on an engine, it is not uncommon for the top pilots to rebuild once a season. The serious local pattern guy (like me) can expect two or three seasons from a well maintained engine.

I have rebuilt dozens of engines, both two and four strokes. I find the average club flier does not know when his bearings are going bad. I have checked engines on the flightline on a typical summer evening and found several engines with bad bearings. The owners usually respond with "it runs ok". By the time bad bearings manifest themselves in a poor running engine, other damage can occur, like the big end of the conrod wearing excessively because the journal is flopping around. In the case of a riveted bearing, a rivet can let go and destroy the inside of the engine faster than you can say "what happened?" My test for checking bearings is to see if there is any play in the prop tips if you try and move them fore and aft when the engine is cold. If you can detect a wiggle, time to replace the bearings. The bearings in a four stroke tend to last a longer because they are not exposed to raw fuel as in a two stroke. As others on this thread have commented, the alcohol absorbs moisture and causes the bearings to corrode. After run oil used after every session will help protect the bearings. I find my YS pattern engines don't need after run oil until the end of the season, but I run them almost every other day throughout the flying season.

I also rarely change the front bearings unless the engine was involved in a crash, the front bearings seem to last forever.