The UV rays from the sun do the real damage, the heat accelerating the process. So measure the time between touch ups in seasons rather than years.
As the surface just begins to lose its mirror shine, rub it back & apply two new coats of varnish, to replace both layers (the one that was damaged & the one you removed by sanding). If you leave it until the varnish begins to crack or peal you risk the underlying wood being damaged by weather. A good varnish job done properly should last ten years before needing to be replaced.