I have 2 floating over a spray bar in a 4g. In my case, it takes them about 3 weeks to start covering the surface, but usually by that time they have couple of baby plants. I replace parents with off-spring and the cycle repeats.
It's one of my favorite plants. So easy to keep happy, and a very fast grower. Does require a good bit of maintenance though, or else it will literally take over an entire tank (however big the tank is).
Water sprite is practically indestructible, I have it in all my tanks. It positively thrives in low-light conditions. Someone initially gave me a few stems a few years ago, and since then I've sold lots of it, or tossed it out by the bucketful.