Change It Every Three Years
If someone--or multiple users--exercise on the treadmill on a daily basis in a way that generates strong impact, such as running, then the belt may need to be replaced if it begins to display signs of wear such as a tear or if it is curled up on the sides, which can happen after three years of such use.
A Replacement May Not Be Needed For a While
If your treadmill habits consist of casual walking a few times per week and limited pressure is placed on the belt, your belt may last longer than the average runner's treadmill belt, which is about three years.
Based on the intensity of your weekly usage, it is up to you to assess the maintenance needs of your treadmill. Extending the life of your treadmill by replacing the walking belt as needed will ensure not only your safety, but a positive exercise experience as well.