i have a fence full of mustang grapes (unknown species). I don't press the grapes because i like to bottle a bunch of the juice for nonalcoholic purposes, it takes around 60lbs steamed to get 3 gallons give or take.
the times i have used a press took about a 5 gallon bucket with stems to get a little under a gallon. i would estimate it was between 20-30lbs worth. i did some wine with those last year.
it was hobo wine though, so i just used some bakers yeast and and a couple of two liter pop bottles. i might as well have called it cellblock 13 because it taste like it was done in a toilet.
now that i have been introduced to some proper techniques i will be making some better stuff this year.