when to flower??

hello,

there are 2 plants one is unknown strain and the other one is a super bud.

the tall one is the unknown and its 18" tall
the super bud is only 8" tall but very bushy

i am using 120w cfl for them both and just wanted to know how long i should wait before i start the flowering stage.
i might also get a 250w hps before flowering them..

Whenever they're about half the height you want them I would say cut the lights back. How old? They look about ready

I would flower now but you can anytime, even from seed. It look looks like the 120w cfl has done a good job so far but will become less and less effective the bigger the plants get. The 1st plants I grew (durban poison) at least tripled their veg height in flower. Also it looks like you may have (not sure) one indica (shorter) and one stavia (taller) plant. If so the indica may only double but the sativa might triple in height. Take a look at the topping, LST, super cropping and lolipoping tutorials if you want to manage the plant size and lighting issues.