low data mode on demand

You did not mention that you wanted this to be an automatic response, you said that you would like to turn it on when you reach certain limits. This is how you turn low data on and off. There is no method to do this automatically. You can provide feedback to Apple if you would like to see this feature by providing feedback here, www.apple.com/feedback/iphone.html

By going into Settings>Cellular>Cellular Data Options>Data Mode>Select Low Data Mode.