we accepted a reservation (or "Pre-approved it") more than 48 hours ago. When we accepted it, a note popped up saying that the guest still has to submit payment information. It's been longer than the 24 hours and it still says "awaiting payment" and those dates are blocked on the calendar so other guests cannot book.
This happened to me a while back. I don't recall the exact time but believe it was 48 hours for guest to arrange Payment. If they don't Airbnb will remove the stay and free up your calendar.