610,
There are quite a few variables.
The amount of actual power you have, how loud you play it as a percentage of the maximum potential, the amplifier topology (whether Class AB, G/H or D), the inherent efficiency of your system design and installation execution, type of program material, EQ settings, the amp/hour capacity of your battery(s), how you charge your batteries when the boat is in storage and the condition of your batteries. Results may easily vary by +/- 100 percent with seemingly the same equipment. With an honest 1000 watts of Class AB power with a single group 24 battery in good shape you could assume the average play time at rest at close to max volume to be an hour.

David
Earmark Marine