Hello SuMsHaDy, you can clone from a plant in flower yes. Will it shock them? Only if you throw them under anything but 12/12.
If revegging, it may take around a month before the clone would be useful. The shock of coming out of flower lasts anywhere from a few days(depending how long in flower) to a few weeks. The revegging takes another few weeks.
If you plan to clone from a plant in flower, be sure to put it under 24 hours lighting. Anything else and it will sit there saying "What the fuck, I'm in flower damnit, there's still a dark period!!!", and hairs and baby calyx's will form.
@ mynameispot
Best time to clone from a mother plant, is during veg(never flower). Every 30 days give her a haricut, and poof, 30+ clones if you have a decent sized mother. You can snip a few here and there every week if you want. But always in veg, and atleast a week or two after the last haircut(trimming down).
Best time to clone for a mother plant.... Take a clone in veg, when you out find the sex, if it's female keep the clone. Top the clone on its fourth or fifth set of leaves. Clone the whole thing below the topping point. Make the new pre-topped clone the mother.