10 weeks.
If the show is too long (BB6, BB7, BB8), we are left with a bunch of average housemates because all of the big characters are taken from the show earlier in the game, meaning we have quite a few weeks with dullards. This also reflects on the winners (Anthony, Pete, Brian) and the only reason we end up for rushed finale interviews it because six housemates just isn't managable in the time allocated - we even get a double eviction, making the first two interviews 4-5 minutes long at the most.
In my opinion, 11 housemates in a 10 week timescale (just like in BB5) would be the best bet for BB9.