I take my Demco tow bar apart once each year and inspect for any damage. I then clean everything and lube things to insure that it is working fine. As long as everything feels tight and shows no sign of wear, it should last as long as my toad. The tow bar is in a location where it will pick up a lot of dirt and debris as you are going down the road.
If you don't feel confident in your mechanical abilities I would go with the manufacturer's recommendation on service intervals.