Are you a US national? Do you have Global Entry? If not a US national, are you travelling on an ESTA, a visa, for a purpose which might raise questions at CBP, or do you routinely experience secondary inspections at CBP?
As a general matter, as you are connecting, if your flight is late, it will be EK's responsibility to rebook you so that is not a concern. What is a sensible time is largely determined by the CBP factors above.