If you are using 2 sets of batteries before you go out and buy the charger you need I would do a few test on them after an average day(for you) fishing. Make sure they are topped off , test them and then go fishing, When you get home test them again to see how much they have gone down. Now you will know how much power will be needed to top them back off.
way3slow could be right about how long it could take but it all depends on how big the batteries are, what condition they are in and how much you use them. Just a good way to find out for yourself is to test them by using them how you use them and then decide how you will need to charge them. Good luck.