Hrm, yes I suppose it could be level dependent, perhaps that is the part in the wiki that is incorrect. The wiki is pretty good about capturing those, but is not currently showing this spell as level-dependent.
http://wiki.project1999.com/Group_Resist_Magic
Wiki says 36 minutes, not level dependent, but it is lasting ~27 minutes at level 50.
Not the end of the world as far as bugs go, but either the spell duration is wrong or the wiki duration is wrong.