Agreed, winter is the time. The advice is to take them back to about a third, but mine had got well out of hand and the previous owner had let it take up all the top of the greenhouse so it shaded everything. I chopped it right back to about two foot, I am not greatly attached to it so wasn't too worried if it killed it. Far from it, I am now training it along a three foot high frame with wires across it and it is growing like stink, though I don't know if I will get any grapes this year.