For rhythm, try just learning a lot of different tunes. Listen to CD's or MP3's and try to figure out what the guitar player is doing and keep trying to copy it.
After a coupla hundred songs you should be able to do just about any type of rhythm. If you can't figure out the chords to a song by just listening, get the chord charts or tabs.
As far as practice time goes, it all depends on yourself. How hungry are you to learn guitar, how good do you want to get, what are your goals for the future etc..
Basically, the more you practice, the faster you'll be able to get better... but you don't want to overdo it.

Personally, I never had a regular practice schedule. I just played when I got bored or wanted to learn a new song. I always figured if you don't enjoy what you're doing there's no point in forcing yourself.
However, there were several instances where I was forced to learn a lot of new stuff. ( like when joining a new band.) So, I had no choice but to practice every day. In the end I found that it was pretty beneficial cause it forced me to learn stuff I would have avoided otherwise.