I use 17 gauge BHB7 at 45lb in my Pure Storm Tours. It's usually good for 6-10 hours before it starts to stiffen up.
I cut it out as soon as I notice a change in feel, even though the string stays playable for a while before it bottoms out. The string is relatively cheap, so I don't see any reason to push it.
USRSA MRT - K Six.One 95 16x18