I don't have a hard/fast rule. Depends on the specific juice (some of them are very powerful, others more delicate) and the batch size, and things like how big the bag of frozen fruit is. I keep it simple, so using a gallon of juice and a 2x 3-lb bag of blueberries, for instance, is definitely something I've done. Pretty sure that was a 3.5-gallon batch; ask AmandaK how it turned out.
I do tend to stuff 25C and 28B with fruit character and then blend in "straight" cider or mead to taste later. Easier to go too high and blend to get the balance I like than finding out later I'm low on fruit. YMMV
if that 100% blueberry juice was just slightly cheaper, I'd drink it every day. the juice, not the melomel.