Uk Cheese How Long Should I Veg For?

Hi guys i have some uk cheese cuttings they have been under a 600w lights for nearly 3 weeks now and are about 1.5 foot tall how long should i give it before i change the light to 12/12? Also im planning on using 3 600w lights when should i introduce the 2 extra lights to the grow room?
 
Hi i have 15 in about a 12 by 12 foot room. Last time i did 6 in a 1.2 tent and got 12oz so im being greedy this time and want to at least double that im using an connoisuer bid bud and overdrive
 
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