I signed up the last week of October and it tracked all my purchases through December 31st (you can always check you balance on your "my ebay" page).
On Jan. 3rd at 9:21am I received an Ebay BUCKS certificate that was good upon arrival and expired on Feb 2nd.
Once I won the Monroe Pharmacy Receipt and went to pay for it, my ebay BUCKS cert was already in place, and I just paid the balance. The easiest reward program I've ever seen!
BTW... They even had a double points day after Christmas and I got an extra five bucks for that.