It doesn’t matter whether you use a loofah, a pouf or a sponge in the shower, you should change them regularly.
In fact, according to a dermatologist in the US, you should change a pouf or loofah every three weeks and a sponge every six weeks.
Of course, if they start to smell or become discoloured before this time, you should change them right away.

Dr. Cameron Rokhsar said on Rachael Ray’s morning show that these items can be a breeding ground for organisms which cause infections and rashes.
The alternative is to put them into the washing machine for a good clean. This can apparently make them last up to three months.
Hat-tip: Apartment Therapy