can you put it straight into 12/12 from 20/4? & how much will i get if i start?

You can put it to 12/12 anytime you want, but you will get better results and a higher yield (Note: if you provide your plant with proper light and have enough grow space) if you Vegg the plant under 20/4 for 4-5 weeks. By this time your plant will probably show pre-flowers signaling that it is ready to produce bud at any time when you change the lights to 12/12. Your plant will average 3-4 feet if you do this method by the time of harvest (also depends on genetics and your grow methods obviously) and if you give it enough root space like a 5 or 7 gallon pot of premium soil with the proper nutrients, you will end up with a nice fat harvest.

Ideally you can enter into the flower phase anytime you deisre. There are many methods of growing i.e scrog and sog methods with smaller amounts of time of vegetation. I usually vegetate 4-6 weeks prior to entering into flowering phase. what else I do to help the plants adjust the dramatic change is I keep the vegetating plants that I will enter to flowering in the dark for about 3 days. Helps your plants aclimate to the change.