Mother Lights

So i have never grow indoors yet and i have a question.

How much light is adequate for keeping say 3 monther plants?

Here are my three mothers under (4) T-12 x 24".

I will keep them under this as long as I can. I don't need tall just bushy.

4 2 footers eh. I was leaning towards a 4 bulb 4foot T5 set up with 54 watt tubes but i'm at the end of my budget. My space will be around 4' by 5' or 6' for the mother/clone area. They will need to produce cuts about every 6 weeks. Thanks for the replies guys