Formula for how long will it take? Water weighs ~8.33 lbs. per gallon. A BTU is the energy needed to change the temp. of 1 lb. of water by one degree F.
You can back into your answer roughly by knowing this formula-
heat (Btu/hr) = Flow Rate (GPM) x 500 x Temperature Difference (degrees F).
The500 accounts for water at 8.33 lbs. per gallon x 60 min/hr. You still have a few missing variables unique to your application, more details including your environmental data and application goals would be needed to get a really accurate answer.