A couple months ago I brewed a Chocolate Hazlenut Oatmeal Stout (CHOS) and used Hazlenut extract. I was unsure how much to use in my brew and exactly when to add. I decided to only do a primary fermentation and let the beer finish before adding the extract. At about 10 days I pitched 1oz. of hazlenut extract and let it sit for another 6 days before bottling. I like the results, there is a good hazlenut smell to the beer after the pour and the hazlenut finishes nicely after the chocolate, it is not overpowering.
It ultimately is a big experiment, you just need to try and adjust for your next batch if it isn't what you are looking for since we all have different tastes. I have read that the earlier/longer you let the extract sit in the fermenter the weaker the flavor with be.
With all that being said I would try 0.5oz-1oz of extract and let it sit for 6-7 days before bottling. It worked for me, hopefully you have successful results also.