1. First add a column with the start date of the execution
2. Add a second column which calculates the difference between start and end timestamp in the preferred time unit using the DATEFIFF() function (see DATEDIFF support page)
3. Calculate the average difference in time, using the AVG() function you already mentioned
4. Add the average duration to the start timestamp, by using the Datetime modification functions. Use here the same time unit as used in step 2.

As outlined by Jan you need to calculate an average on a timeframe between two acvitivites. DATEDIFF is one option to do so but there are several other options to do this based on your needs.

Check out this help page on it: https://confluence.celonis.com/display/PQLdevelopment/Throughput+Times

Best,

Kevin