I have added the zest (grating from the outside, very little white, mostly the orange outside skin) of 3 medium sized navel oranges to an amber'ish ale. It wound up being about 1.5-2 oz's or so I think. Added this with 10 mins left in the boil. Fermented for 2 weeks.
At first (before 3 weeks in bottles) it was pretty bitter at the start of each sip, but now around 3 weeks it's mellowing out fairly nicely. I think 1-1.5oz's is fine for a 5g batch, just ensure you give it a few weeks to bottle condition and mellow out a bit.
Can't say as I really taste 'orange' when I drink that batch now, but there's something about it that's nice. I also added about an oz of fresh ground coriander seed. Rickards White'ish...