Hi,
You can pretty much start flowering whenever. Remember that in flowering, the plant will double or triple in size! Use that knowledge to determine when to flower. So if you only have 4 ft of room... you might want to start flowering at 1 and a half feet. You'll never know unless you can research your particular strain/seed manufacture and see what they say about the average size growth during the flowering stage. Or use this as a trial run... make sure you have a before and after height so you know for next time = )