veging

I've got a 2.4mx1.2m tent with 2 600watt hps lights with 8 plants (4 under each light). how long should I veg these for to fill the space?

Also before I was thinking just before putting them into 12/12, take some cuttings and have them veging for 8 weeks under cfl or hps, but I want to keep the wattage low. what size tent would i need and what size light? id want them to be a nice size, so soon as the plants in the flower room are harvested I can move the veged plants into the flowering room.
 
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