How much protein powder is it safe to drink after a 2 hour workout?

Protein is necessary. You should aim to get about 20 g of protein daily. Realizing that some of that is in your diet, be careful not to "overdue" because the metabolism of nitrogen in protein can burden your kidneys. Protein is fine after exercise, as it keeps your metabolism up and contributes energy that you have burned. A balanced approach (bar with protein, carb, and lipid) might be better, however.