If you have a light directly over your tank, it will go from covering half the tank to covering 100% of the tank in a few days. If you thin it back to just a few leaves, it'll be back to 100% in a month or so. It's a great way to keep nitrates down, but it can be a pain after a while.
If you can find a source for giant duckweed, it grows at a more managable slower pace, but I don't know where you'd find it.
(Message edited by jennewt on January 25, 2006)