Many will say that ~1-2 months for refrigerated harvested yeast although I have some jars of yeast that I have harvested and used and re-harvested for 3 or 4 generations over the last 6-8months. I know this will not go on forever, and I will replace this yeast with fresh yeast here soon, but as long as aseptic technique is upheld, autolysis does not occur, and you dont get something funky, you should be ok for a few months.