There is a lot to goes into it, that's why people have so many different opinions on it.
Everything has a fatigue life... crank shafts, ring gears, axles, etc...
The harder you stress anything (either more power or more weight), the shorter the lifespan of the part will be.
My 3.23 gear (heavy load) sheared the pinion off... didn't even make it 50 passes behind my 3700lb mid/low 10 second tuneup. But the STOCK axles survived.
Lots of guys have broken axles without hurting ring gears.
It's not one thing that fails, or one weight, or one power level, its a combination of things.
Generally though just from what guys have done, regular 3300lb race weight is good down to around 10 flat or so. There are certainly failures at that point, and certainly people that have gone quicker... but it's a good rule of thumb that will PROBABLY ensure a reasonable lifespan out of the parts.
For the record, my clean break on the pinion killed the sprag too. I highly recommend that you ALWAYS at least look at the sprag if you hurt the rear axle/u-joint.
