My best guess is 10 minutes. Excruciating long if you need an ambulance!
Emergency vehicles are subject to all routine traffic laws/signs/lights. That's my honest guess. Hope this helps. Sorry if it sounds like I'm being a jerk. It's not intentional.
Nights they need time to get dressed if they were asleep. Otherwise from when they are paged out should be just a few minutes. 10 or so. Who does the dispatching? Local or county wide centralized.
Also, in Kennebunkport Maine they have to clarify Main ST, from Maine ST, from North Main ST. The County dispatches from several towns away. They want to help you, honestly, but which direction should we go?
Just for laughs, consider in the states I've run, there's no law giving emergency vehicles the right of way. The law says everybody else must yeild the right of way. Subtle differance but crunched ambulance is automatically at fault.