I voted 25%, but probably it is about 1/6 of the time. It's a 16th-level party these days, and rogue has a ring with three more charges of anti-magic in it. The party uses it mainly defensively. When we went through oh, Deep Horizons I think it was called, the official WOTC module by Skip WilliamsI believe, there were beholders in there that almost toasted the party (level 13/14 around that time) with their coordinated anti-magic cones and desmodu/troll fighter servants.