Kitchen sponges are tiny, but they’re one of the dirtiest items in the whole house. As a thumb rule, a normal household should replace sponges every 1–2 weeks, and scrub pads about every month, depending on how heavily you cook. If you cook daily and wash lots of oily dishes, lean towards the shorter side.
You can stretch them a bit by rinsing thoroughly after each use, squeezing out extra water, and letting them dry in an airy spot instead of leaving them soaked in the sink. Some people microwave damp sponges for a few seconds or dip them in very hot water to kill germs, which helps for a while, but it doesn’t magically reset an old, worn-out sponge.
Signs it’s time to throw it: it smells even after washing, it’s changed colour badly, it tears easily, or bits start coming off on dishes. At that point, you’re basically rubbing old food and bacteria onto “clean” plates.
Sponges are cheap compared to your health. Keeping a small stock at home and swapping them regularly is a simple habit that keeps the kitchen more hygienic without much effort.