If US rules require you to declare an emergency when the 45 minutes state is reached you have to follow their rules and declare the emergency. You can't just say "We're Euro operators, we are going to ignore your regulations because we have our own." It's as simple as that. Think about Checkerboard's car driving example.
As for the question when an EU aircraft has to declare an emergency, the answer is whenever either applicable domestic (e.g. EU-OPS) or foreign rules (e.g. FAR) require it to do so.