You seriously don't want to keep it longer than a week. And that is only if you don't clean it with bleach, or toss it in your dishwasher when you do the dishes. If you continue to clean that dish sponge with either bleach or by tossing it into the dishwasher while you wash the dishes, then you can keep it indefinitely. The bacteria that builds up and lingers in a dish sponge is disgusting. And if you don't clean it, you will not want to use too long. So either switch to a dish cloth, or keep that dish sponge clean please.