Should I veg it longer? (pic included)

Its almost 28 days since sprout, I was planning to veg till 16th march, should I veg them longer?


heyheyhey.jpg

stretchy one got burned, never found out why because it happened before I fed them for the first time, soil has no ferts in it.
 
around 2-2.5ft, 3ft is max limit for my grow room.
first two is kush strain and others are WW.

I wonder what is average yield for that size with 400w.
 
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