Not often enough! But I do try to pick one cupboard/pantry section to go through at least once a month ... some sections come to my attention more often than others. Right now I'm not baking much, so that shelf has been neglected, and I know I need to go through it.
We are actually in the process of reorganizing our whole kitchen due to the influx of wedding gifts (we can actually use them all!) With both of us working and the age of the kids, it's not possible to just get in there and do it all in one swell foop, but we're getting about one cupboard a day done.