Where do I set the time period how long sucessful process instances are kept in the monitoring section?

The "system default" value is set in some system config file which I would've thought would be one of the standard ones (like custom.properties) though I haven't touched these in forever.  Any chance your question is answered here? https://docs.appian.com/suite/help/21.4/Managing_Process_Archives.html

Edit to add: this link appears to contain the exact answer: https://docs.appian.com/suite/help/21.4/Data_Maintenance.html

Also just to note, each process model can be set independently, though sadly each process model's value must be hardcoded for some reason.

To summarize, I have to proceed as follows, right?

1. Stop the Tomcat Application Server
2. Stop the Search Server
3. Stop the Data Server
4. Stop the Service Manager
5. Edit the property AUTOARCHIVE_DELAY within APPIAN_HOME/conf/custom.properties
6. Start the Service Manager
7. Start the Data Server
8. Start the Search Server
9. Start the Tomcat Application Server

To summarize, I have to proceed as follows, right?

1. Stop the Tomcat Application Server
2. Stop the Search Server
3. Stop the Data Server
4. Stop the Service Manager
5. Edit the property AUTOARCHIVE_DELAY within APPIAN_HOME/conf/custom.properties
6. Start the Service Manager
7. Start the Data Server
8. Start the Search Server
9. Start the Tomcat Application Server

I'm out of date on how to stop / restart the engines for a local install, but assuming your steps 1-4 and 6-9 are the correct steps and ordering, then this sounds good to go ;-)