The answer to this question will vary based on which sealer you use and which kind of stone you have. Some types of granite are much more porous than others, and the more porous types will need to be re-sealed more often.
As a general rule, you need to re-seal your granite countertop once every 1-3 years. Again, this will vary by sealer (the more expensive ones tend to last longer). If you use your counter every day, it should not take long to recognize when liquids are no longer beading on the surface.