The usual time to add citrus zest is with five minutes left in the boil, similar to aroma hops. You just scrub the fruit pretty well, then zest it and dump that in. You can of course add it later too (e.g. zest the fruit, soak the zest in vodka, and throw the peel and that liquid in as a "dry hop" once fermentation is over, or even at bottling time). I haven't tried all these methods myself, so I can't speak from experience about how they are subtly different.
A standard amount to use would be the zest of one or two whole oranges in a 5-gallon batch, depending a little on the freshness and type of the orange. Randy Mosher recommends using the zest of a single sour orange (also known as bitter or Seville) in many recipes. So I imagine a good starting point would be one orange and one lemon. But if you want just a hint, you could use half of each (though lemon are of course smaller than oranges).
