Grape growers tend to prune several times through the season. There are so many variables depending on the type of grape, the weather, growth and other conditions that may change from year to year and month to month. But, pruning yearly is necessary for vine health and crop. Check out the various extension service papers via google on pruning grapes where you can learn a lot more than I can offer.
I'll soon be picking grapes in a vineyard that's been neglected for 5 years. Only about 20% of the vines are still alive and only about 20% of those have fruit this year. The lack of pruning prevents light from evenly ripening the grapes, the sugars aren't high enough in the shaded fruit so only some of it will be usable for anything but feeding the birds.