I would go around week 5 because of the space that you have you dont want them to get to big . Other wise I would answer grow the plant until it shows signs of maturity AKA the calyx with "hairs" and then you have an option you can keep vegging till the plant shows signs of pre flower OR you can basically flip the lights right then and there and start the flowering process . It's always recommended that you let the plant mature before you force it to flower this way you dont stress it out extra raising your chances of getting "Hermie" plants . I hope you find this information useful and that it helps you understand the "when" to flower a cannabis plant . Other then that I'd say your in great shape and doing a good job . -Happy Growing!