The circle lasts a full 60 seconds, but both practical effects of Guardian Circle (The healing effect and turn undead effect) only work for the first 30 seconds. Even lowly Level 1 skeletons aren't the least bit bothered by it after the 30 second mark.
(This might be a separate issue, but also Necromage seems to have no effect on the time when it should be making the duration 90 seconds, but even the visual aspect only lasts 60 seconds for me.)
Running 3.0.10.