My meter says 275 CFH and our appliances use ~200 KBTU/hr.
You could run one of your appliances and clock your gas meter to check how many cu. ft. per minute it uses.
It takes 830 BTU of energy to raise 1 gal. of water 100F. If you can do this in 6 minutes you must have put in >8300 BTU/hr of power, so you may have used >8.3 cu. ft. of NG.
"A cubic foot of natural gas gives off 1000 Btu, but the range of values is 500 to 1500 Btu". My gas company claims that 100 CF of their gas gives off 1 therm, but who's to know?