You can only donate a certain amount to another player, determined by your VIP level.

When you reach the limit, you must wait until the donation rollover at the end of the month to be able to donate to the same person again.

On the 1st of the month, donations reset and you will be able to donate to people you'd previously maxed out your donations for. This also clears out your "Donors" tab, and any pending donations.

All donations must be collected within 48 hours, but at the end of the month, all pending donations are removed regardless of age.