We offer a very generous 24h period and have been known to extend that period for up to a week. All we ask is that you contact us first for help when you need it. We do try to respond very quickly to messages!
If you post a negative market review and then ask for a refund right away, we have no way of determining if you are a malicious competitive developer trying to ruin our apps reputation to further your own.
It would also be nice to know what you didn't like about the app so we can improve it for others. Of course, this information is not mandatory for a refund.