Not often at all. I don't recall seeing one in at least the last several years.
Magic Kingdom is the park most likely to see the different phases of capacity closures, and the most frequent times are Christmas-New Years week (especially Christmas Day and New Years Eve), Spring Break week when it falls close to Easter, and July 4th.
I was told this morning by the CM when I returned the internet kit that Monday is the day. She said that WIFI will be available at each site and they will no longer have to give out the kits.