Answer: Grape vines are usually cut back in late winter (February or March). The pruning is usually quite severe, but how much/where to cut depends on which training system you are using and whether you have new vines, mature vines, or overgrown neglected vines. Basically you need to establish a good framework and then routinely trim off nearly all of last year's growth leaving just a few buds for the coming year.
Here are some illustrated instructions on pruning grape vines you may find helpful. You may need to cut and paste the complete url into your browser in order to make it work correctly.
Good luck with your grape vines!
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