Contingent on ur space size . 12 5 gallon buckets is alot under a 600watter is u dont manage them somehow.. scrog...lst...super crop.. its all about wut ur going for. u can effectively flower clones right out the gaits...if ur tryn to stay small...veg them for 8 weeks and theyll be monsters when ur done flowering.
I go around 10-12" or after 5 weeks. It can depend on any height limitation in your grow room also. Most if not all strains will grow between 2 and 3 times their veg height. I look for the plant to have alternating fan leaves (not straight across from each other. You know the plant is mature and ready to flower for sure with that. Peace!
Damn with 1200w spread out over a 8x6 id veg im for least 5 weeks, while topping them and lst ing them as much as possible to get least 8 canopy shoots per plant. switchn them over when their around 24in to 12 12.