Harvest:
Harvest grapes when they are full-colored and sweet-flavored usually from late summer to mid-fall depending on the variety. When grapes are ripe, the stem will brown and slightly shrivel. Grapes do not continue to ripen after picking.

For more information on growing Grapes, check out this great guide from Harvest To Table:
https://harvesttotable.com/how-to-plant-grow-prune-and-harvest-grapes/