Quarterly or annually a survey should be sent to all customers asking them their general opinions of the support they receive. You cannot ask for specific information when you do periodic surveys because customers only remember their last one or two incidents. Periodic surveys are not a good measurement of how your organization did for the year. Another limitation of periodic surveys is that survey responses will not be specific to each Support Group or person assigned the case. Customers do not really know, or care, what group handles their request.
Here is a great White Paper on Implementing Service Level Agreements. See https://www.givainc.com/wp/it-help-desk-software-cloud-whitepapers.cfm