Two things are likely causing the problem;
1. In your Capture Prefs, you likely have scene detection enabled. (It is by default) Turn this off, and you'll get lengthy captures, for the entire length that your tape runs or until you turn it off.
2. In those same prefs, you can go in and specify the length of capture from one frame to one hour, your choice. Set this up for 5 mins, and that's the length your captures will be.

Video Capture>Options>Preferences>Capture Tab.