If you search online for answers to this question, you'll find that everyone has an opinion! Some say every time you fill with gas, some say with every other tank, once a month, once every X thousand miles, and so on. Unless you receive qualified professional advice to the contrary, we recommend following the manufacturer's advice – it knows best what the product can do. Many fuel injector cleaners are intended to treat a given amount of fuel (100 gallons, for example). However, the internal combustion engine is a complex mechanism, and no two are quite the same. If you're dissatisfied with the results you're getting, try a different brand next time.