Depends on the capacity of the battery and the output of the charger.

Is your "marine" charger also a "smart charger"?
Some marine chargers are just rust resistant (not rust proof) versions
of cheapie auto store chargers sold for 3 times the price.

If it's a smart charger, it will charge in three distinct stages. The
first two pour a lot of juice into the battery to bring up cells fairly
rapidly, and the last is a "float" stage that supplies just a trickle
of current to keep the battery topped up.

If you have a smart charger, you should be able to run it until the
batteries are fully charged and then leave it connected (assuming you
check your boat every few days or so) to keep them that way.