James1990
Stranger
Stranger

depends all on the plant dude,and how long it takes to root. also, itll take longer for a satvia to flower than an indica.

perhaps you want to start yourself a thread for these questions, especially because i don't really understand what you mean. cannabis takes 6 to 16 weeks to flower, just like km says. a 12/12 light cycle will kickstart the flowering *process* (which is why i said "right away"), so you should be expecting to *finish* 6-16 weeks after that, depending on the strain.
BUT, if you take an unrooted cutting and put it under 12/12 (which is what i thought you meant originally), you risk it not rooting AT ALL, in which case you're also not harvesting anything in 6 OR 16 weeks. right?

rooting your clone will take anywhere between 5 days and 2 weeks. after that, if you switch to 12/12 right away, you should be harvesting (average) 8 weeks after you make the flip. is that the answer you were looking for?

oh, it'll *flower* right away, whether it'll *root* for you is another question altogether. personally, i'd give it 18+ hrs of light until it shows roots, and THEN start your 12/12.