You don't need to use bentonite if it clears on its own. I use bentonite as a last resort if time doesn't clear it up. I have a batch of cider from this year's harvest that cleared itself up perfectly in a few weeks.
If you must use bentonite, you can use it any time you want, but I'd wait for it to clear up as much as it can by itself so the bentonite has less work to do. Get the cider really cold (put it in your fridge if you can, or on your porch if it gets cold where you live), then mix in the bentonite. It works much better when it is cold. Stir it up several times the first day, then let it settle out.