730 is probably the CCA (cold cranking amps) for the battery. There is another rating written somewhere on the battery giving you the A/H or amp hour rating. It will probably be down around 100. The bigger the battery, the higher this number. This is the rating that you are concerned with. Theoretically, a battery rated at 100 A/H will support a load of 10 amps for 10 hours. But since most batteries are depleted at around 50%, you would get around 5 hours of use. Deep cycle batteries can go much lower than a car battery. Those numbers also determine the length of time required to recharge the battery. Using the example, that battery depleted to 50% would take 5 hours to recharge at 10 amps. So with all that bull, find your A/H rating, divide by 7, subtract a few hours, and that will tell you how long it will last. Approximately.
Fair Winds and Following Seas
Bill H. PTC USN Ret
Chesapeake, Va