How much duck you run completely depends on how your body is built. What ever stance gives you the comfort and range of motion needed is what you should run. The bad side of a lot of duck is that the toe and heel side of the board could start to behave too differently. Though there are boards made to work better with the duck stance by having different shapes on the heel and toe side. Smokin's Awesemetric and Gnu Park Pickle for example.
I think if somebody ran a -90/+90 stance it'd be a little hard to ride!
Honestly when you thnk of what you're trying to acheive with duck (i.e. a natural athletic stance) you don't see football players with their feet on 45 deg angles. I would say use as little angle as necessary to get a comfortable stance for your particular body.
I use -9/+9 or -9/+12 depending on the board. Lots of people seem to like 15/15 and a few use more than that, some guys ride with 0/0 and it works for them.