For good production grapes should be pruned every year during the dormant season. For the major yearly pruning you will want to plan on doing it in the spring before your vines have much growth on them. I do mine in March. That being said, in the summer their growth can be rampant with canes up in the trees, covering other bushes and blocking walkways. Sometimes it seems that they will reach right out and grab you. I have found that a little summer pruning is beneficial to keep them under control.
When I prune in the summer I look for canes that are growing in directions that I don’t want. For me that usually means they are blocking my paths or climbing a tree. On the cane I want to cut I look for the bunch of grapes that is forming and from there go out at least one set of leaves past the bunch to cut it off. If it doesn’t have a bunch of grapes forming then I just cut it wherever I want to. This is something I do all summer long and it worked well for me. My vines are beautiful and healthy. I get masses of grapes and my vines are not taking over my whole yard.
Visit my other post on training grapes over an arbor.
