I went 2 years ago. We got the coach from our hotel on I-Drive. Pretty sure it was just over an hour, manily due to several other pick-ups.
You have the choice of visiting the astronaut hall of fame too on the way - that's worthwhile. You'll be lucky to spend all day there though, unless you drive. I think our coach left before 10am, so after the HOF stop-off, we were at NASA for noon.
There are several cinema shows, but you'll probably only have time for 1.
you tour the complex on coaches, but can get off and on them and spend as long as you like at each point. Some good photo-spots.
check out 'lunch with an astronaut' if it's still going - for around $25 each you can have a full lunch with an astronaut. it's in a smallish room, only around 80-100 people in there, and the astronaut gives a little speech and vid show. we were soo lucky, as the day we went Al worden from Apollo 15 was there!. he spent almost an hour talking to us, and gave out signed pics (no other autographs allowed) before posing for pics and chatting to everyone. they usually have shuttle astro's there, but 3 of the apollo guys do odd events. have a look on-line. well worth the extra little bit.
think our coach left 4ish, back to I-Drive by 6pm. A long enough day, and it does give you enough time to look around.
any questions feel free to ask.