I've used a red dot for years on my handguns when hunting deer. Never had one run out of juice and hunt for 5 days straight. When I'm setting I have it turned off with the gun in my lap. If I see a deer I turn it on as I raise the handgun. When I'm walking (very slowly) I always have it turned on and ready to go.
At the beginning of each season I replace the battery.
I know that doesn't answer your question for the length but some you could do is put in a new battery and leave the RD turned on and time it. One thing to remember if its colder they won't last as long.
Turk