Time to flower? A little guidance...

This is my first grow.
I shall outline my basic setup for you, and it is basic.

2 vegging four-way plants
1 in straight compost
1 in perlite:compost 1:2
4 18w 1350 lumen cool white fluoro tubes

I'm renting a basement room with nowhere to ventilate to, but it's cool enough and has a draft with the window open 24/7.

I've been using a 7:7:7 nutrient compound

They're in their 5th week now, after some topping and LSTing to keep them stubby.

Seem to be doing well to my untrained eye, although I plan to re-pot them before flowering.

I read that alternating nodes are a good sign of maturity ready to flower, but I fancy them being a little bigger.

Is it time to change to 12/12?
Thanks for any help.
Peace.
Bath.Bud

Well, they look healthy. If your going to re-pot them, I would do it a week or two before you begin to flower, in case of any transplant stress. Other than that, getting some more powerful lights to flower with would be a good idea; like a 250-400w HPS. You can wait as long as you like before you start to flower them, but they are ready if you choose now. However, keep in mind that they will stretch several inches before they produce buds after being put on 12/12.

Thanks Dylan,

I've been looking at a few HPS setups, but I'm not sure if I'll be able to keep the temps down.

My space is 80cm x 40cm x 140cm, and they are in 12" pots. If the light stays high and the plants dont creep too tall, might I be able to get away with no i/o ventilation for a 400W hps? The door will be open quite a lot and I have a fan too.

I'm bidding on a 400W to come through in about 2 weeks, sounds like a good time to start flowering.

Thanks for your help.

The pic with your fingers in it, I think I see a pinched location that has not come back yet.
Wait 2-3 weeks after the last pinch before flowering.
They look healthy though.