This can vary depending on many things. The users experience with prepping data in Tableau Desktop or with Tableau Prep can grealy change how long it may take them to prep the data set. Another factor is how the data is being stored. Is the data clean or is there also cleaning taking place?
Also user experience with such things as Custom SQL or other options for particularly tricky data sets can change how long it takes to properly prep. Can you provide some more information on what is being done with the various files above so that I may be able to provide some suggestions for decreasing the time it takes to prep the files each month.
The extracts could also be created programatically with the Extract API 2.0 : https://onlinehelp.tableau.com/current/api/extract_api/en-us/help.htm Then published at the end of each month to Tableau Server. https://onlinehelp.tableau.com/current/api/rest_api/en-us/help.htm
Hope this helps!