Bob,
Battery life will vary widely. Looking at just the numbers and loads imposed upon a marine battery, there is no reason to expect any different life than say a car battery.
The big variable between a car battery and a marine battery ( as far as life expectancy) is battery maintenance. Typically a car battery is monitored on a daily basis by the car itself. As long as the car is driven on a regular basis and for more than just a few blocks, the battery maintenance is pretty much self fulfilling. Thus most car batteries have a generally long life.
Boat batteries are sometimes the most mistreated, especially in smaller boats using smaller engines generating less recharging amps. Compared to a car, some boats are only driven blocks before being shut down and then restarted later, again and again all day or all weekend. This type of operation is not conducive to long battery life. This can be compensated for by having a 110volt maintenance charger wired into the system so that upon arriving home, one can just plug it in and the battery will be brought up to a full charge and kept there until the next time you are ready to go.
To answer your question directly, the best thing would be to take it down to your local auto parts store. They can check the specs, put it on a tester and tell you right away if the battery is in good shape or needs replacement.
