new indoor grow need to no how early to flower

im gonna start an indoor grow and its a stealth grow so i need the plants to be only growing a foot and a half. so when should i put them into flowering.

depending on strain, they can double or triple their veg height throughout flowering.
Indica dominant strains will generally stay shorter than sativa dominant.
Look into LST, so you can keep your plant at the right height without stressing it too much. I would veg for half your total height, then use LST if they get outta control during flower. MH bulbs also keep the plants shorter and bushier, and to a lesser extent so do the other penetrating HID lights. If you use anything other than HID lights, using additional light below the plants will help keep them short and bushy.
Good luck