In the C20 ( B ) i am not sure there is much to be gained by doing more than a minute or so . I was told that the MD 500,s which did many thousand engine off,s , when their engines went in for service they were absolutely perfect
And they were often shutdown straightaway or after a few seconds of auto . Personally i would do 2 mins in morning to allow me to finish coffee and ciggy but 1 min on last shut down so could get a drink . So average of 1.30 ............engines fine .