It depends on factors such as what height restrictions you have, what type of strain it is etc.In the main expect an indica to stretch twice, a sativa three times or more ( although I'm guessing you're not tackling full sats first grow?

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Most plants from seed will not become mature until 5 weeks or more so ideally you would want to wait at least this long before flowering .
I personally flower my indicas at around 15 inches for a finsihed height of about 3 foot but I only have 4 foot to play with in total.