how often to change the belts in mercedes e 320

belts should be checked at 30,000 miles- if they have been adjusted properly, they should last 90,000 miles- but check them regularly and look for wear-

The belt should be inspected and condition recorded with each scheduled maintenance. Usually every 60-k miles but I tend to look for and replace when the smooth part of the belt has a high sheen and or several small hairline cracks. If the ribbed portion has any cracks or splits, replace immediately!