If you are dedicated to it, you can really tone up in a month. If you swim hard every other day for an hour the with painful ab work outs on the off days you'll be good. Gradually work up to longer and longer swims and more intense ab exercises.
You just have to know how to push yourself to really get the most out of your time, and have a chat with a school coach about good ab workouts. I can't really describe any on here.