The machines that I am intimate with have never stated outright what those figures are.....and I doubt many will ....unless it.s a MIL Spec proviso (ie Blackhawk)
The figure that gets bandied about by the techs that dig into these hard-to reach places is 30mins....
I have known Bell mediums to sieze within 15mins ...others to have sustained serious damage (scrapped GB) in as little 3 mins...
There are many factors as to whether a gearbox holds together or self-destructs. Things like rate of fluid loss, payload being carried, ambient air temp, transmission design, fluid being used .....all the way to the tolerances within individual boxs (no two are exactly alike)
I have personally had two trannys puke thier fluid out....once from 55psi to Zero in about 30secs (fortunatly was just in the hover....whew!! and another it dropped at a rate about 5psi /min (where I elected to carry on to first landfall....no damage...a/c returned to service)
It is my personal evil monkey lurking in the closet when it comes to helos....You might have 2 hydralics, 2 engines, 2 generators, 2 inverters, instrument redundancy......but there,s only one tranny....take care of it!