I would say it depends heavily on the level of musicianship and the intentions of the project. If you've got professional seasoned musicians who have been doing this a while and have little to no day jobs then there's no reason why they couldn't be ready to go in a week or two. But like many people if you have normal jobs to work around I would say that if you're dedicated enough you should be able to put something together in a few months. You just have to be clear with everybody on how intense or relaxed you want this project to be I guess.

I’m going to assume a couple of things from your post; one, that most of your members are of similar age and experience, and two, that you’ll be playing cover material. I reckon a six-piece can learn between 3-6 songs in a three-hour rehearsal and any six experienced musicians meeting for the first time likely know at least a dozen viable songs in common. So, if everybody is playing at a near-pro level and the band doesn’t use charts on stage, you could memorize three fifty-minute sets in three weeks or less. (if not, somebody’s not doing their homework, heh) The problem is how to keep the material fresh, since you’re not gigging; you can’t rehearse more than three hours’material in three hours, and what you learn on the gig sticks better anyways. (of course, once you’re gigging, you’re free to concentrate on intros, endings and new material at rehearsals) I think you need to schedule some gigs you can split with another band that may also be short of a full night’s material OR go to two-a-week rehearsals temporarily. Also, make sure you don’t have players in your group with psychological problems regarding gigs; I’m in possession of an eight-page, single-spaced classic-rock set list from a band I was in back in the nineties that never gigged because we, “didn’t have enough material”.