How often do I need to clean the bag of balls? I have been just rinsing the bag off with the balls inside every couple of weeks.

Most filtration systems have the water flow through a filter pad to remove most of the larger debris before water flows to the bio balls. With this type of system, the bio balls remain clean therefore they do not need to be rinsed or only need rinsing a few times a year. Since one of the main purposes of bio balls are to be a surface area to grow beneficial bacteria you want to rinse the balls in old pond/aquarium water or de-chlorinated water. If you do not have a filter pad before the bio balls than you may need to rinse the bio balls more often.