I set GSM to save some entries when PC has been idle for 70 minutes, but I noticed it gets triggered way way earlier. I have the "Add Windows Task" option (or whatever it's called) enabled, and upon inspecting the Task itself I noticed the actual checkbox that makes that mechanic work is not enabled, only the 70mins time is set correctly but that's ignored if the checkbox is not enabled. So I'm guessing GSM doesn't enable the checkbox programmatically when creating a "on idle" task.
EDIT: and I guess also the "wait for idle for:" needs to be set to "do not wait"
GameSave Manager utilises the schtasks.exe commandline tool (which is part of Windows).
Perhaps some changes were made to this and wasn't documented / I missed it?