I think it depends on the level of the choir. It's possible to have a very productive rehearsal in less than an hour. Most of the time, we just repeat stuff over and over, when we should move on.
I was in a rehearsal that was 20 minutes, and we practiced 5 songs for an event that was to take place later that day! Since everyone had the music, we mainly practiced timing and directing cues.
I think if a Minister of Music can keep the rehearsal on schedule and minimize talking, a lot can get done in a short amount of time.
I think new songs should be practiced for about 10-15 minutes per session, working on the difficult parts first.
....You know, even if the choir isn't that experienced (their voices are lacking), you can still have a rehearsal in about an hour. The key is just picking songs that they already know fairly well and singing those, while they rehearse newer or more difficult songs. A song that is already learned shouldn't have to be rehearsed for longer than 5 minutes, even if you have to do restarts. (Most songs are less than 5 minutes.)