It depends on driving behavior and gear oil for the most part. Clutch type as well but if your a good driver clutch type really wont matter.
When i switched to a twin disk I went through 3 gearstacks sheering the teeth off of 3rd and 4th within 3 months, that was at over 450whp and 400wtq. I was using 75w90 in the trans. I switched to Valvoline 85w140 gear oil on the next stack and it lasted 9 months even at roughly 550whp 450+wtq 30psi on occasions and 450whp daily. Never broke the gears. Also tried not to slam into 3rd or 4th quickly either but did a couple times and it survived. Never broke that trans before i switched to the 6 speed. Also never broke a trans on the track on slicks. Slicks provide much more cusion on the gears than street tires do.
If your doing a simple t25 or t28 setup you really have nothing to worry about. I never broke a stack on my t25 and t28 setups and slamming gears all day long and heavy clutch as well. My buddies nx has been boosted for going on 4 years now and same trans, 50k miles of abuse, slamming gears all the time, races it all the time, always spirited driving when the opportunity comes. Its on a t28 at 15psi now and no issues.