The simple answer here is: 100%
You digest 100% of the protein you eat, no matter the amount.
What most people really mean when they ask this question is how much protein is optimal for health and recovery?
The answer to that is within these guidelines:
*Eat at least 20 grams of protein per meal
*Eat 0.5-1.5 gram per pound of body weight
*Eat complete sources of protein
I know that's quite the range, but it really does depend if the person is utilizing resistance training. If that's the case, I would go closer to at least the 1 gram per pound of body weight. If not, I'd at least aim for the 0.5 grams per pound.
Give plenty, train hard, and eat well!