I used 8 ounces in an imperial stout, and it's definately noticeable (you could probably get away with half the amount). Here is what I did. Ground them up slightly in a coffee grinder, roasted in a 350 oven for about 30 min just to ensure they were sanitized, then put in secondary. Since this was an imperial stout they were there for over a month.
For less of a chocolate flavor a couple weeks should be fine.