Heres how Id do it.

Volts= Joules/Coulomb
Amps= Coulombs/second.

For a 5 amp hour battery, that would be 5 * 3600 (secs in an hour) = 18000 coloumbs

at 30 amps 18000 Couloumbs (storage)/ 30 (rate) = 600 secs. Roughly 10 minutes.

Thats how Id do it anyway. Some1 is now bound to bring in an complex efficiency factor on the battery, but thats the basic physics.