When I was using the ads manager, I used 30 ad credits on a 3-day daily budget campaign, but for some reason it only let me select 2 full days (48 hours exactly) and when the ad completed it only used 20 ad credits, and the remaining 10 were not refunded. I am sure that this is definitely not intended behavior, as a can also do a lifetime budget for 20 ad credits
If you’re expecting a refund, then it can take a few days to process.
as for why it limited the number of days: ¯_(ツ)_/¯