It depends. Some people make a tea and don't use the whole leaf in their tanks. Some add the leaf for spawning so the male can make a bubblenest below it, but the leaf eventually does become waterlogged and sink on its own. You can generally keep the leaf around until it starts decaying molding or rotting or falling apart in some way.
If you're using a leaf in a very small tank you probably don't need to use a whole leaf. The leaves I have release quite a bit of tannins even with smaller pieces.
I'm not an expert on IAL but i'm sure someone will come along to answer your questions better then I can.
Here are a couple websites with interesting info though.