Don't use cherry extract. You'll be very unhappy with the results. Cherry extract tends to smell like cherry but taste like cough syrup. Also, are you going to a LOT of chocolate flavor, or just a noticable hint? This will determine how much cherry you should use.
If you're looking for over the top chocolate flavor, I'd probably go with a full lb. Some people think this is too much, but I think it's perfect to get a good choco flavor. You can add half of it to the boil with about 10 minutes left and the other half in secondary when you add your cherrys.
If you're just looking for a hint of chocolate, I usually use around 6oz. Again, I think half and half always works best.
I also suggest using REAL cherries. I understand they are expensive, but it's worth it. Frozen cherries will also work. You can probably even get away with cherry pie filling if you check the ingredients list carefully. I usually use 1lb of cherries/gallon but that produces a pretty strong cherry flavor. If you want to chocolate to be noticable of the cherry, then use around 3lbs.