You start earning credits again the next day.
FYI - the trick with the Custom Challenges is to make sure you log off of Reach when the credits are set to be added. Go play another game during that time, or log off XBox. When the custom challenges credits are added offline, they don't affect the credit cap.
I usually do the custom challenges as the last thing I do for the day. That way they are added while I sleep.
[Edited on 12.20.2011 12:37 PM PST]