How long would I need down there?
I quess that would depend on what you're fishing for and how lucky you are.
March and April are generally the months for the biggest blow outs. Sometimes small craft warnings are up for a week at a time. It is also good permit and bonefishing then as well during the calms, with big tarpon in Florida Bay estuaries and resident juveniles in the Keys as always.
There will be sailfish around as well.
Kayak rentals abound in the Keys as elsewhere in Florida.