We re-use them as much as possible and the 'hardest part' is then determining when they do finally need to be replaced... Usually we do so only if/when something 'sticky/gunky' has now stuck to the bag itself, or someone in my family has thrown something stinky or sharp in it and has now 'punctured' the bag. (Unfortunately on the last one - it being damaged/punctured) we don't realize it until it is too late and then have to wash out the tras can itself!