pc grow question. when should i start flowering..

hey,

i have a server pc case. i have 18inches to work with. how tall should the plants be before i start flowering cycle. i want to try and use the whole 18iches, but i know its not easy. it depends on the plant n stuff but i want to see what u guys think.
im growing THC bomb, bubbleponics..

~thanks

Well, you have 18 inches all day to work with or it it 18 inches starting at the top of the planter? Where are the lights? When a plant goes into flowering, it can double and triple in size. If you have 18 inches to work with-barring any restrictions about planter and lights-then if you grow your plant more than 9 inches, you are going to be very crowded long before harvest.