The Dispatch Mean Time to Complete Report will provide you with the average time it took for dispatches to be completed. The report uses the 'Time' column of the Dispatch History Report for the given Dispatches based on the filters you've chosen.
For example, if I pull a Dispatch History report for the past 7 days and choose completed during that time frame; then run the Dispatch Mean time to Complete report and you would see the average time converted into days.