If you use high quality impregnating sealers like DuPont Bulletproof sealer, you should have to reseal your granite every three years. If you use a cleaning liquid called Revitalizer also made by DuPont, you will not have to reseal your countertops ever. A gallon container of Revitalizer only costs $39, which will lasts 2 to 3 years depending on the size of your countertops.
Read more about sealing granite countertops here.