That background consists of several keyframes on a few effects. If you wanted to make the animation last longer, you could duplicate the keyframes along the rest of the timeline, or you could scale the animation by expanding the group of keyframes, or you could use time-remapping...
There are a lot of ways.
As far as why the background animates for 5 seconds: These animation presets are just examples; they're starting points for you to modify them for your own purposes.
I think that you might benefit from beginning at the beginning and working your way through the materials here: