You should dig that out from extensive back testing, divide up the day every fifteen minutes and find out on average you can make, then you can stop when there is a drop off, for me it is 60 minutes in ES, but I add extra fifteen minutes for a variance when not having a good day and possible get back to even.
Typically you know that you are shot when you start trading for the sake of trading...Chasing the market higher or lower is another bad sign...