Originally Posted by swag

40 minutes is sufficient, if your inbound is on time. If you are delayed, maybe not.

If you arrive late, you can ask the agent at the arrival gate to let the departure gate know you're on the way, but I wouldn't expect them to hold the flight. Just this past Sunday at DFW, I watched a late-connecting passenger coming in from London arrive at the gate for the last flight of the night to STL 8 minutes before scheduled departure. Despite the jet bridge still being attached, they didn't re-open the door.