How much of prune juice per day for an adult to help constipation?

My father has constipation problem, his doctor told him prune juice will help. The question is how much should he drink a day, one big glass/day? I heard that it's more effective when drink warmed prune juice? Thanks in advance!

as we called it where i worked, the geriatric cocktail. About 90-120 ccs heated should do the trick..bout 1/2 to 1 cup.