As is the case with many questions, the answer to how often to wash your gym kit is: “It depends.” If you are someone who doesn’t really break into a sweat and stands around gossiping, your kit may last a few visits before washing. But if you work out hard, it will need washing every time. Many modern sportswear fabrics demand a low-temperature wash; however, that won’t get rid of the bacteria that can thrive on sweaty clothes. If you wash at lower than 40C, it is worth adding an antibacterial laundry cleanser. You can usually find them with washing liquids and powders in supermarkets. It is either that or crank up the heat.
It doesn’t matter whether you exercise indoors or outdoors: the kit will still get dirty. In a gym setting, you can pick up germs and dirt from other people using the machines or mats (think colds, flu, gastrointestinal infections, skin infections). If you exercise outdoors, you will pick up mud, bacteria and stains from the natural environment. Either way, what you wear needs a clean.
Rather than leaving stuff in a bag overnight, it is better to wash your gymwear at once, as this reduces the chance of bacterial multiplication. You have to do it at some point anyway, and if you wash it straight away, it is ready for your next outing. Finally, don’t forget to wash your sweaty towels and your water bottle: if this can go in the dishwasher, put it in, otherwise it will get smelly.