One thing to note is that the higher your settings, the shorter your clips will tend to be. But - when you go to the video editor, you can crank the settings back up, and it will display on the current spec, not the ones you recorded under.
Generally speaking, you will find more cars and pedestrians around will reduce you clip time. You can turn vehicle density down in single player, or put traffic to low or off in multiplayer. Peds off too in multiplayer will help.