There are exceptions (during special occasions, some comedies and dramas will run 45 or 75 minutes to try to drag in more viewers (and allow for more ads), but network shows are designed for 30, 60 or sometimes 90 or 120 minute blocks. The last is almost always a movie. Comedies are generally in 30 minute slots, and dramas are generally in 60 minute slots. "Reality" shows are generally 60 minutes or longer, but it depends on their ratings at the moment - (I think. I don't watch those things myself.).
Local channels are similar because they're almost always tied to a network or they show reruns of old network shows.
The news is on multiple times in a row because: 1) "Rush Hour" is 3-4 hours long in many cities; 2) It's very cheap to rerun local programs; 3) There isn't network programming until 8:00 Eastern Time and syndicated shows are (likely) more expensive than rerunning a local show; 4) It gives the channels an opportunity to brag "Highest Rated Local News at 5:00!!!!!111!!" even if, say, only 30 people are watching.
Programming in the Eastern and Central time zones are usually tied together. Some show will be on at 10:00 Eastern/9:00 Central. The Pacific time zone seems to get programming at its own time slot. I'm not sure how things are generally handled in the Mountain Time zone.
[link|http://www.zap2it.com/|Zap2it] lets you look at customized TV schedules for the US. (Scroll down).
As you've noticed, we have lots of channels. Most of them are junk (shopping channels, pay-per-view, dozens of movie channels that show the same movies, 30 "music" channels, etc.), but they generally keep to the same type of schedule as the networks because that's what we're used to.
HTH a bit.
Cheers,
Scott.