From what I've found, in order to get 1.050 SG (a middle-of-the-road standard apple juice SG) out of a standard can of concentrate (treetop, vons brand, ralphs brand) one must use 2.5 cans to get a total of one gallon. These are the only brands I can attest to, but I suspose it's pretty close to this for most brands. If all else fails, just do some math, one gallon is 128 fluid oz.
Primary:Russian River Redemption clone, Kelly's Melomel, Graham's English Cider 22-23
Aging:Public House Dry Stout, Procrastination Porter, Mr. Brown Ale, Westvleteren 12 Clone, Mead, Duvel Clone, Graham's English Cider 6-21, Belgian Draak Strong Ale, Fig Melomel, Acerglyn, Restorative Tonic Metheglyn