Same business day. My guess is that Drs offices would be busy today because of people who got sick/hurt yesterday, and may be down a staff member or two.
Depends on the issue. Concerns related care - I call back ASAP. Questions about money & payments - within about a day. Insurance issues - I take longer because the answers are never found quickly.
It depends on the doctor and the reason. When I would call my OB and request a call back it was same day, usually either during lunch or before they headed home. When I call my RE they usually get back to me within an hour. My PCP is usually within next day.