I think it's something you have to experiment with. I recently brewed a breakfast stout (5 gal) with a full 12 oz bag of beans using cold steep ( added at flameout). The coffee flavour was strong but not overwhelming and the beer did quite well in a local competition with the homebrew club. If you do the math, you would essentially be using full-strength coffee in place of brewing water if you use 4 full bags of beans (48 oz) for a 5 gal batch. You'd obviously want something less potent than that, and I used a quarter of that strength for mine. Do you want the coffee to be just noticeable or a significant contributor to the flavor profile? If you're adding it post fermentation you have the option of adding it it increments till you're happy with it.
One thing I firmly believe is that you should go for the best quality coffee you can find and use a light roast. Dark and medium roasts cook all the flavour out of the coffee.