How often does a granite countertop actually need sealing?

Less often than you are usually told. Test it rather than working to a calendar: put a tablespoon of water on the surface and wait fifteen minutes. If the stone darkens where the water sat, it needs sealing. If it does not, leave it alone. A dense granite in a normal kitchen commonly goes two to three years between coats. A lighter, more porous stone around a sink might want it annually. Sealing more often than the stone needs wastes an afternoon and does nothing.