my plants are about 8" tall can i switch them to 12/12 or do i leave em to grow?

Generally it's best to wait until they're sexually mature.

You need to consider how large your growing area is vs. how large the plants will be once they've finished flowering though. Depending on the strain, plants will usually increase in height from 2x to 3x in the course of flowering.

So if you're growing a sativa dom. plant and flower her at 8", you're likely to end up with a 2ft high (or so) plant... which obviously isn't very big

my plants are bbcheese all females and they have been in veg for 2 weeks if that helps

Generally it's best to wait until they're sexually mature.

whats sexually mature mean and how do i no when they are?

does the canopy of the plants fill the space ur growing in?

not sure what you mean but my plants are 8" tall and 13 plants under 3 x 600w hps/mh lights and i have about 4 foot headroom to grow in

I veg'd some bluecheese for like 4-5 week and I got 2o of it. I'd wait until they get a bit bigger mate.

id definitely wait till there a bit bigger mate........like what someone else said, they normally double or even tripple in size once you start flowering so if I was you I would wait till there at least 12 to 14 inches tall

good luck m8

3 600w with only 4ft of headroom ? That sounds pretty crazy , what are your temps in there like?

Sexually mature is when they show balls or pistils and start growing with alternating nodes rather than symmetrical ones.

sorry its 5ft and part of the room is 8ft high so lots of air space and i have 2 x 10" fans blowing in and a 12" fan blowing out
and as for sex im still vegging and these are from female seeds

you got some nice ventilation there but be careful of positive pressure in the room pushing the smell out

maybe lose one of the 10" fans then you're have negative pressure and all the odour will go through your filter

even 5ft of headroom isn't enough for 600's really, maybe if you can get the lamps RIGHT to the top of the 5ft, just - 18inches airspace from bulb to plant, the plant itself, then the height of whatever they're growing in won't leave much space if any

8 ft is plenty of room

i put my plants into flower at about 15"-18"

hi Steve good luck with it, you might want to see if there is a thread here somewhere on how many plants to maximize your grow area and your amount of light and then make some clones to do that since all is still in veg happy days