Is This True?

I was told, once you place a MJ plant in 12/12 flower, it will grow and stretch for one month, then it's done growing and will only fill and grow bud for the second month, females of coarse. Is this right??

basically.... Really slows down vertical growth after maybe on 2 or 3 weeks aswell.

That's correct, in a general sorta way. After placing a plant in 12/12, you can expect vertical growth for about the first 3 weeks. If it's a heavily indica dominant plant (short and squatty, with thick, wide leaves) it probably wont do much in the way of vertical growth. If it's a sativa, or a sativa dominant hybrid, you could expect it to double or even triple in size within the first 3ish weeks of flowering. After that, yeah, it's all bud production.

pure sativa's may behave differently - from what i've read anyways, some take a long time to kick into flower
but i do short veg's, and that's pretty close to your description - it takes a while for a plant to mature from seed before it will go into flower