Cartoon Network seems fine in that department. I notice Johnny Test and Total Drama airing quite a bit on the network, but I don't think they do weekly marathons of those or most shows, and they have a pretty varied weekday schedule.
From what I notice, Disney's alright there as well. Sure they mostly play their various kidcoms across the day, but there seems to be some amount of consistency on there.
Nick though... whenever I turn to that channel, I swear if SpongeBob or iCarly aren't on, it's a fluke. If that's what is doing fine for them, then great, but I doubt that every kid in the country wants the same five hours of both shows everyday that Nick puts on. A little more variety wouldn't be too bad.
I don't pay attention to the digital cable channels even though I have them. From what I can tell on NickToons Network though, Avatar fans should be happy since they must replay the entire series at least once a week.