You can do this with VBScript and the WScript host:
Save this as "press_enter.vbs" and run it from the CLI with "wscript press_enter.vbs"
Dim WshShell, FSO, secs
Set WshShell = WScript.CreateObject("WScript.Shell")
Set FSO = CreateObject("Scripting.FileSystemObject")
secs = 10800
kill_file = "C:\Users\Public\Documents\kill_switch.txt"
Do While true
WScript.Sleep(secs * 1000) ' this is in milliseconds, multiple by 1000
WshShell.SendKeys "{ENTER}"
if FSO.FileExists(kill_file) then exit do
Loop
As you probably guessed, the program runs forever until it detects the existence of the file "C:\Users\Public\Documents\kill_switch.txt"
You can also kill the program by opening Task Manager and killing the "Microsoft Windows Based Script Host" which will be listed under the "Background processes" section...
Remember that if you stop the application with the "kill_switch.txt" file, you will need to remove it before you start the application again. Perhaps it is best just to kill it with Task Manager...