How long to heat a 25000 gallon inground pool with gas heater?

{Sigh} I hate English units.

A BTU is defined as the amount of heat required to raise the temperature of 1 lb of water by 1 deg F.

The basic relation is

Q = mcdT

Heat = mass * heat capacity * change in temperature.

A gallon of water weighs about 8 lb, so you're talking about 500,000 gal * 8 lb/gal = 4 million pounds.

The constant c for water, by the definition given above is 1 Btu/lb-deg F.

So

Q = 4 million pounds * 1 Btu/lb-degF * 32 deg F = 128 million BTU

Note: 500,000 gal should be changed to 1250 gal or 3.2 million BTU?