You can veg for as long as you light the plant wont flower untill you switch it to 12/12.
I run 18/6 if i have had no problems during veg.
For flower i do 12/12 and it depends on the strain and your conditions when its ready.
Im vegging some Blue Cheese under cfl at the moment and it was stretching.
I got a reflector and moved it about 2" away from the canopy and it sorted it right out.
At 4 weeks its beautiful.
I use that Powerstar loveintheganja sent a link for and it works a treat.
Also you can buy a heavy duty timer from a lot of grow shops now that will take 1000w.