I recently brewed a Mocha Stout and I used 4 oz of cocoa nibs into secondary for 2 weeks. Taste is good, but any less would be an issue..... any more would also pose an issue.
What is your serving temp?
I noticed that if I serve the beer any lower than 46DF, I start to lose the chocolate and coffee flavor. The warmer it is(48-52) the more pronounced the coffee/chocolate flavors are.