The amount of time you should leave a foam cannon on your car can vary depending on a few factors, such as the condition of your car's surface and the type of soap you're using. However, as a general rule of thumb, most manufacturers recommend leaving the foam on your car's surface for approximately 5-10 minutes.
During this time, the foam will begin to break down and dissolve any dirt, grime, or contaminants on the surface of your car. This is known as "dwell time." It's important not to let the foam dry completely, as this can make it more difficult to remove and potentially cause streaking or other unwanted effects.
After the recommended dwell time has passed, simply rinse off the foam with a high-pressure stream of water, working from the top of the car down to avoid streaking or water spots. Then, proceed with your regular car washing routine, such as hand washing or using a pressure washer with a high-quality car soap.
Again, it's important to note that different foam cannons and soap formulations may have slightly different instructions, so always consult the manufacturer's instructions for your specific product.