It looks like the latest update to 10.5.7 seems to have broken OSX time limits in parental controls.
Several reports are emerging:
https://discussions.apple.com/thread/2018656?start=0&tstart=0
http://mac.bigresource.com/OS-X-Parental-Control-Time-Limits-Broken-in-Snow-Leopard–J67KN3EAv.html
http://forums.macrumors.com/showthread.php?t=790056
But this is an example of a much older report (last year):
http://forums.macnn.com/t/370370/parental-control-time-limit-not-working