It depends on how dark you want the water. If you are just using them as preventative measures, you could go with as little as 4 inches of leaf (my leaves are about 7-8 inches so this is half) for 2-3 gallons and still see a difference in activity, but not color the water much (depending on the leaf, some do more than others!). This way they last a long time, especially if cost is a factor.
If you are breeding fish, or have a sick fish, I would add a whole leaf (broken up if you want to release tannins faster) to that same amount, not exceeding 2-3 leaves per 25 gallons. Males often build bubble nests off them while they are floating.
Of course some people also simply don't like the leaves in their tanks at all, so they use hot (100 degrees) water to brew it as a sort of tea, and then once the water is cool add that to the betta tank. Similar to the extracts in a link someone posted above. Too much trouble imo, plus you lose the benefit of other organisms like snails and shrimps eating off the plant if you have them and the fish getting to hide under/around it, but as I said, some people don't like the leaf in the tank at all.