It depends on how big the batch and how strong the malt flavor. This really is something the you have to do to taste. I've used chocolate extract in the past but have limited it to about 1oz extract per gallon of beer (assuming you have single strength extract). If you bought a 5 gallon kit but end up with a smaller than 5 gallon yield (due to trub or whatever) then adding all the extract at bottle will make it stronger than they intend.
You could always experiment... Add half the extract and bottle half the beer, then add the other half to have "double chocolate". If you think something like that would be to your liking.
FYI, I never cared much for the flavor of liquid extract, I started using coco powder (same kind you bake with) and having that added to the end of the boil, and present during primary fermentation. It will mostly get left behind when/if you rack to secondary. I like the flavor that adds better than when I used the liquid flavoring added at bottle. But that's my preference, your taste may vary.