I do a lot of re-design in order to bring sites into SEs. Since this is a somewhat dynamic process, I normally contract for 12 months. This gives enough time for all of the engines/directories, including DMOZ. Plus it gives you time to tweak a bit.

Also, you do not get reliable statistics with shorter time periods. If you want to analyse the search terms your visitors used to find the site, then you need at least 9 months to get good data.

hth,
Alex