stock power (~280-290) to low 300's (i.e. 310-320), short bursts or lots of semi...plastic bushings will last quite a while.
I built a mechbox firing 1300+ rpm, hooked it up to a large 9.6V battery and held the trigger down until it broke (basically one long full auto burst). Just for fun and too see what would happen. It lasted about 3500-4000 shots before the bushings failed and the gears stripped. The mechbox (more so the cylinder) was really hot.
The plastic bushings that survived seemed a bit loose (i.e. the axel holes were worn and the axels would jiggle in them)...the ones that failed cracked and the axels went right out of position, leading to the gears crashing/stripping.