depends on how tall u want it to end up... i start off with no more than a foot of veg .. ive had plants almost tripple in size during flower mode....
to keep it compact i would get ur lights as close as possible ( if it doesnt make ur hand burn ur not going to hurt the plant, if ur using cfls to veg u want to lower ur light about a quarter inch off the top of the plants..and when u top it wait until u have at least 6 tru leaf sets i count the one closest to the bottom, once topped just dip it if u have clonex or a cloning gel. if not ive always had much success by scraping the bottom of the stem (lightly about a half inch...then i put it in a plain glass of water about three quarters of an inch....change the water in the cup at least once a day...in about 5 days u may start seeing roots on ur cutting but it sometimes takes longer! anyhoy wait until ur plant gets a foot tall and go to 12/12 and flower it... top it for better yield ....good luck