If you've got a lot of light and CO2, don't buy a lot of this stuff. It'll get overgrown in no time at all! I'd hate for you to over buy and then this stuff take over the whole surface area of you tank!
LOL! Okay, didn't know that was the look you were going for. Make sure to keep checking the frogbit. Don't let the roots touch your substrate. It'll start burrowing and the floating part of the plant will start to grow differently. If you've got other plants below that need light, you might need to re-think the set up. In any case, best of luck!