Helo!
I have been hear that is 1/10 or 10%.
Anyway its the same than other questions, no one can confirm this kind of %, we only can make a "study" of it but never will be a real result.
Anyway i want to give u great trick for this bosses and for this crystals in particular.
This Crystals have 1day to expire (or 24h), but if u place this crystals on the depot this time will STOP.
This crystals inside the depot only will lose it durability when u stand next to depot, while u hunt/sleep this crystals will be infinite!
I hope i helped u.
Gleemody