If you're connecting via the USA, then you probably need around 2 hours to clear Immigration and Customs and get through security again for a connecting flight, depending on which airport it is.

If you miss a connecting flight through no fault of your own, then the airline usually get you onto the next flight and look after you with meal vouchers, etc.

Each airline has its own 'gateway' in USA so it depends on which airline as to which connecting airport you use.