Coffee + Porter can be a wondrous thing! I tried a brew called Espresso Break at a bar. It was a vanilla porter fortified with espresso vodka up to about 9% abv. I took the AHS vanilla porter and added Van Gogh espresso vodka to it in a 3:1 ratio. It made for a fantastic dessert. Damn, I'm going to have to brew that again.
Anyhow, I'd suggest adding it right before kegging/bottling. That way the flavor isn't lost during boiling or fermentation. How much to add is anybody's guess. If you keg I'd say pour a normal pint first and slowly add the extract to your beer like with a dropper to gauge how much is needed. If you bottle then use whatever directions (if any) are on the extract box in something else to get an idea of how much to use.
Primary - Beer
Primary - More Beer
Secondary - Still More Beer
Kegged - Yet More Beer
Planning - A Bunch of Beer and some mead
I always forget to update this thing, so the above should in general hold true.