Gotu Kola is often confused with Kola Nut which is a caffeine source.
In my experience Gotu Kola kicks in within a few days if not immediately. Tolerance does form but very slowly. I've used it for a couple of months now and the effects are still there. I remember one anecdote where Gotu Kola was still effective after over a year of use.
I use Solgar brand Gotu Kola extract 3 capsules once or twice a day. It's the only brand sold here locally. I used to buy Gotu Kola from iHerb but they changed the law here and now all orders have to be cleared through customs. In my experience Gotu Kola works pretty well both as the whole herb and an extract.