Oh yes. I have been using IAL for over a year now on my bettas. They take a few days to tan the water. I think a new leaf takes 2 days to sink. They float at first. But they can leech tannins as soon as a few hours depending on certain conditions.
I recommend them all the time because they help boost a bettas immune system. They are good to always have in the tank, there's no such thing as too many IAL, the darker the water the happier the betta is, you can crush them up in a food processor and make holistic medicines.. Fun fact, the Asian groups who use bettas to fight actually condition their scales with IAL to strengthen them against external injury- while also providing excellent immune systems to ward off infections later from the fights.