Depending on where the wood furniture is located, clear finish or varnish will have to be reapplied every so often. Outdoor furniture takes a beating from wind, water and sunlight, so it will require refinishing sooner than indoor furniture. Even indoors, however, varnish begins to wear or flake away far more quickly than wood stain or paint.
By most average estimates, clear wood finish will only last 2 to 3 years before refinishing is needed. Unlike stains and paints, varnish doesn't protect wood from harmful UV light. That, as much as air and moisture, takes its toll on wood furniture.