Every day you hold onto it, it'll deduct another $1 (or $2 for HD) from the sell back price. You can't hold onto it for as long as you want and get $19 back.

But I don't know the answer to your original question about whether it takes off another dollar at exactly 24 hours or if there is any grace period. My guess is there's no grace period though.

http://blog.vidangel.com/category/how-to/