They say it's good until midnight the day of check out - but that wasn't my experience. Checked out and was shopping in the BC gift shop / marketplace waiting for the car service to arrive. I was shopping for DP snacks to take on the plane - but mostly just to use up our 8 remaining snack credits.
After spending 15 minutes making careful selections I was told by the CM that my credits were no longer available. Tried another card - same thing. Went to the front desk and she said that it was b/c i checked out already - ?????? She was looking into when DH appeared at the desk...
Our driver was here and we had to go. DH was not about to let me miss our flight due to unused snack credits, so I had to leave without solving this issue. But I left my beloved WDW on a bad note -

. We will be back in Nov and I will not let the same thing happen again.
The moral of the story is: Don't wait until the last minute to use up remaining credits - or you might lose them!