What should be the debounce time for the diagnosis of short circuit to ground?
SCG is a common automotive ECU fault diagnostic function such as the O2 sensor which is a field replaceable part.

It may be a long time in ms e.g. 500ms but not too long to stall the engine.   

this spreadsheet shows some ECU fault detection parameters that may be useful.

http://dulevo.com/documenti/codici-guasto-5020e6.xls