If you don't need to do any editing, Snagit would work. If you want to do any editing at all: zooms, callouts, etc. Camtasia is what you should be looking at. I would recommend utilizing the free trials to determine which application best suits your needs.
My only question is why the 15 minute limit? Have you bumped up against some type of limit due to a computer setting or memory limit? As far as I know, there is no time limit using either of the software mentioned above