A minimal script could look like this:

Set %MAX_CALL_DURATION%=300
Print Wait for new call
Wait HasCall
Print New call available
Wait !HasCall,StateIsActive
If !HasCall
  Print Call ended unanswered
  Load %LOAD_FILE%
EndIf
Print Call answered - wait for %MAX_CALL_DURATION% seconds
Wait %MAX_CALL_DURATION%,!HasCall
If !HasCall
  Print Call ended
EndIf
Print Max call duration reached - terminate call
HookOn
Wait !HasCall
Load %LOAD_FILE% 

That just disconnects the call after 300 seconds (5 minutes). You can optionally send an announcement after the max time. Just insert before "HookOn" something like this:

Say Maximum call duration reached. Buy!
Wait !HasCall,WavePlayed 

Just save the code in a text file (e.g. "script_max_call_duration.txt"). You can test it by typing "load script_max_call_duration.txt". If that is OK you can enter the filename as "Startup script" - so it will be loaded automatically.