Depends on what your saying. If you take a cliopping and try to get it to root it can take a week to two weeks to root. Once it has rooted and you put it into flowere 12/12 it will start showing it's in flower withing a week. You have to have a viable clone to start with.
well first its gotta root out to come out of shock from transplanting then it really depends on size i would do 18/6 for 2 weeks then its really up to you cause it will depend on how big you want it and how much room you have to grow your plant the rule of thumb is soon as you go to 12/12 for flowering the plant will double in size or more mine almost went triple there hieght when i went into flowering
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?
a mini plant hmmm mabe buy dawrf seeds or just top your seedling early and put it into flowering mabe 4 -6 days after topping so veg it like 3 weeks thats what i would do but you m ight not get a good yeild