Very active fermentation can chew through the fermentables in mere hours. Some would call that "done". Just trust the hydrometer and leave it for a few days past when the readings are stable. Once your process is down you'll hit the numbers like clockwork and you'll also have a lot of other batches going and probably won't even bother taking readings until a couple weeks have passed, if at all. I very rarely take a gravity reading at all anymore, but when learning and dialing in your process, it's the only way to be sure of the results.