How many times do you have to change a canister filter for a turtle aquarium?

I'm going to start saving up some money to buy a 55 Gallon tank with bigger lamps, docks, plants, and most important, the filter!

Before i go out and purchase a good filter,

What filter do you recommend for a 50-55G Turtle Tank?
What size filter do you recommend for a 50-55 Turtle Tank?
If its a canister filter, how much is to replace it monthly?