I too used some orange peel in a Belgian Wit and I think I used it for 30 min or so. 1/2 oz to 1 oz depending on how much flavor you want. I used 1oz. The aroma was fantastic during the boil... Very intense. Once in the bottle, the flavor is really subtle, but it's there.
I think adding the peel to the secondary might be a bit more intense. Personally, I'm not a big fan of "in your face" fruit flavors in my beer.