Engine life is mostly dependent on how well you treat the engine (running, post running, and regular maintenance...kind of like a full sized car engine). If you take proper care of it, it should last through about 5-10 gallons of fuel before you should start looking for signs of needing a rebuild. I know, big window there, but there are some other factors that could terminate the engine's life in a much shorter time span. For instance, the .25 engine in question is a relatively new engine. It has already had a run of manufacturer's defects that lead to shorter engine life. You never know if yours is the next one to turn up a new bug.
Just take care of it, run it as hard as you care to, and be prepared to buy the next monster powerhouse as a replacement...if the need should arise.