How much candied peel can you put in fruitcake?

I think it would. I'm not a big fan of mixed candied fruit, so I usually substitute about a third candied orange peel and for the rest, I use dried sour cherries and cranberries, coarsely chopped, which I've briefly soaked in a touch of whatever liquor the fruitcake recipe calls for (rum, brandy or whiskey). You don't want the extra dried fruit to be too wet, but you also don't want it to soak up moisture from the batter, because the candied mixed fruit does not. An added benefit of these substitutions is that they cut the sweetness of the cake. ;o)