Cards expire all together at midnight. If that's not happening, you might need to change your time zone settings.
EDIT: I guess this might be an issue if you're at the point where you don't have cards expiring every day. IIRC some people who have reached that kind of review schedule have switched to just reviewing once every weekend or something.
With the change in the scheduling algorhythm, the cards should be evenly spread out over time (roughly 10 to 15 a day if you have 2000 cards, or 20 to 30 if you have 3000).
The change is plus or minus upto 1/6th the time decided by the leitner box (random though).
Been doing this nigh on 8 months and still have cards due everyday.
Really minor thing, but it would be nice to see the time zone listed on the my profile page. I didn't realize I could change the timezone until I read it here, but having it listed on the my profile page would make it clear that it's editable if you click the edit profile link.