I normally allow six weeks but much will depend on the band's other commitments and the difficulty of the piece. With Tristan we had five weeks (following a contest and a major concert) but we used our time more intensively than usual and included a couple of extra rehearsals in the week before the contest.
I know this is not my band, and is probably exceptional, but I was talking to Simon Dobson of Zone One Brass at the Areas, and he was regretting the fact that they had only managed 4 rehearsals on Coventry Variations.
We looked at Vizcaya before Xmas then picked up with it on return in January. Never really had a full rehearsal till the week before but that is usual I'm afraid for a lot of bands.
Rehearsing test pieces is down to how settled the band are, and what piece you are playing. I think 4-6 weeks is pretty reasonable.
If the players can all play the notes I think 4/5 rehearsals is all you need really. Obviously the lower down the sections you go, the more time you need to spend note bashing rather than shaping the music. I cant remember when we started on Kaleidescope but I think it was about mid January when it started completely taking over every rehearsal.
I like this topic. I think that a short run up to a test piece is good because it keeps the mottifs and ideas fresh and your performance will always have a positive intuitive energy about it. Keeping tests on the stands for ages has obvious benefits but can make a performance sound to mechanical and over-rehearsed. It's good when you can balance a healthy new energy towards music whilst still feeling the sentiment of playing a cracker contest performance of a good piece that you've grown attatched to over a period of time.