Great advice on here so far. Here's my two Lincolns.
It really depends on the situation, as Ed Fuqua has stated. Another factor to consider is how efficient you are with your practices. Sometimes a single rehearsal a month can be more productive than rehearsing twice a week. If you spend alot of time at rehearsal horsing around, noodling on your instruments, yapping and rambling about non-band related stuff, it will take away from maximizing your time. I'm not saying that rehearsal can't be a time for hanging out with your bandmates. On the other hand, if you just wanted to hang out, you could hang out anywhere. Have your hang out time be after you're done rehearsing, so you can be sure you get the main goal for meeting accomplished.
Learn your parts for a song before rehearsal. If you're doing all originals, this becomes trickier. Especially, if you are in a band that writes together. Songwriting sessions should be times of creative wandering, but rehearsals should be much more disciplined. Write out the changes, or have someone do it for you before rehearsal. Send out or get recordings, even if it's just a riff or vocals over strummed chords. Communicate any song notes such as feel, tempo, etc. prior to rehearsal if possible.
If you are in a cover band, there's no reason you can't learn it from a recording/chart/whatever before you rehearse. Even if it is an original arrangement, learning the changes beforehand will save a lot of time. Learn the melody line as well to help you understand the changes and structure of the song. Once you do this, adapting to an original arrangement of a cover will be much simpler.
Know what you are going to do at rehearsal. You'd be amazed at how much dead air, and how much time is wasted on just figuring out what to play. Get a list of songs to work on, as well as any notes for feels, tempo changes, etc. prior to rehearsal. At the end of rehearsal, you should know what you need to work on for the next rehearsal. Have a goal for what you want to accomplish, and the more detailed the better.
I've been in bands that took 2/week practice for months to get 30 songs down, and bands that get 20 done in one rehearsal. Aside from musical abilities, a thoughtfully organized rehearsal maximizes the limited time you have available. The more you rehearse the better, but you'll only get better at what you rehearse. If you rehearse noodling around, wasting time, arguing, or whatever that's what you'll perfect.