you start your 12/12 cycle when you wnat to force the plant into flowering.

you stay on the 12/12 cycle until you harvest.. some believe that going down to 10/14 for the last week also increases your yeild a bit.. personally, i've never tried it... i might.

you stay on 12/12 until you're done.. about 8 - 10 weeks depending on strain,grow technique, lights... etc.