The way I think about it, you add 3x water to get juice. How strong does the juice taste? What if you add 8x the water? 20x? How much juice flavor are you getting out of it? How does that compare to the flavor of the base beer?
Adding 1/4 cup to 1 gallon is like diluting it with 64x the water. I think at that level it will be subtle if you can even detect it, but I haven't had that juice.
A 12 oz can is 1.5 cups, added to a gallon (16 cups) it will be more than 10x dilution, or about 3x as dilute as intended.
A lot of it will depend on how strong you want the flavor to be, but I think even a whole can wouldn't be out of line.