We just got a granite countertop this summer. The company owner was here for the installation. He sealed it while he was there, and he said probably we'd never have to do it again, but if we noticed stains setting easily, then we should buy some sealer- Home Depot was a name mentioned. To seal it he poured something on the countertop and rubbed it and polished it with a cloth. He also gave us a spray bottle of some kind of cleaner that also seems to wax it a bit.
I just looked it up online, there are several sites. The one I read said that it should be resealed every three years.