My expectation was always 4 business hours. And once I had a good ambassador this was usually met. Early on they also used IM to communicate but they shelved that for lack of accountability and privacy concerns.
1-2 days is too long IMHO, unless over a weekend. In emergencies I would use the phone or, if I am roaming / stuck in a noisy airport terminal I would use SPG Platinum online chat and they will either kick the ambassador into gear or help me there and now.