How many hours will a 100lb propane tank last for a gas fireplace?

86 hrs. and 16 min.

Under lab conditions of exactly 25000 k Btu per hour output with a propane product temperature of exactly 60 degrees Fahrenheit throughout the burn.

That's probably not the answer you're looking for though. Given real operating conditions with variable product temperatures, It would be almost impossible to estimate.

I'll take a stab, and say with an ambient temperature fluctuating between, let's say 10 and 32 degrees Fahrenheit I would guess that you could knock the 6 hours off the total time of the burn. Again this is considering a continuous burn at 25000 k.

My gas grill is 40,000btu's and I have a 20lb tank. I get about 15-20 hours before my tank is empty.

I would figure you to be able to get about 90-120 hours with your 100lb tank.

It depends on how much you use it... but, it should last a long time. Moderate use,... probably 2 months.