If i put a 55 watt 12 volt light bulb on a typical 12 volt battery fully charged, how long will it last approx?

The bulb? Ha Ha LOL.

The bulb will draw about 4 and 7/12 ths amps per hour (or 4.58).
4 7/12 X 12 = 55

Now you need to know how many amps your battery has. Divide the battery amps by 4.58 and you will get the answer. Probably 20 hours or more from most deep cycle batteries.

thanks a bunch, im rigging up for night fishing.