If I recall correctly, I had a few days to decide, but that might also have been because I immediately communicated my hesitation about the lack of a scholarship offer and then waited to hear whether they might reconsider giving me money. (It worked. Leveraging scholarship money is possible, even off a waitlist.) The school I had already committed to was perfectly nice about my decision to withdraw, which was only fair since they got to keep my seat deposit.
This was shortly after the seat deposit deadline. I was then asked to withdraw from any other waitlists, which I did. But I don't think regular admits had to make any such commitment, so an August withdrawal should be doable.