Between 12-15 lbs for a 5 gallon batch. Not knowingwhat type of grape you have is a problem though as some are very high in acid which should be diluted and some should not be diluted as youll throw it way out of balance(PH). Are they wild and can you post a picture of the grape and maybe the vine it grew on so possibly someone can identify it, also tell uswher you live by adding tis to the User CP area near the top right side of your screen. It will be under location once you open that area.
I wanted to post some info on the grape to see if anyone could help i.d. it. I live in Albuquerque, hot and dry, lots of sunshine, and the grape seems to be doing well. It is a red grape with one seed. They are between nickle to quarter size. Let me know if that narrows down the varieties it could be. Thanks.