If it is good, potent, chalk-full-o-chokecherries juice I would add 1.5 quarts to fermenting 5 gal of beer on the 3rd day of the ferment. Before I added it I would heat it to 145 - 155F on the stovetop in a clean SS pan and hold there for 15 - 20 minutes (definitely keep it below 160F or you'll set pectins causing pectin haze) to pasteurize it, and once cooled, add it to your fermenter.
It's REALLY good in stout, and you could add more juice to the fermenter for a stout, prolly up to 2 quarts.
If it's straight press from chokecherries, you might reduce it to 1 quart or even less. I would add, taste, maybe add more.
I use 4 lbs of crushed chokecherries per 5 gal wort, stirred and pasteurized prior to straining/racking to the fermenters, and there's plenty of chokecherry flavor that way.