How long before a Medivac should the pilots be ready?
Medivac pilots (at least the ones in my city) are at the hanger/pad 24/7. 

My home field (KPNE) happens to be where some of the medical choppers are stored and dispatched from. In Philadelphia they tend to not be stored at the hospitals. I have met some of the pilots and one of the crew nurses. Where I am the facility is staffed 24/7 much like a road ambulance facility so that as soon as a call comes in they are ready to fly. The helicopters them selves are kept in a hanger, always fueled and alway ready to fly, on these neat rolling carts so that a tug can pull them out as the pilots are getting things squared away in the cockpit. 

One of the hospitals in the suburbs keeps their chopper at the hospital (which is fairly close to KDYL) and is avalible 24/7 as well. 

I also happen to over look one of the 4 hospital helipads in the city from my home. The pad is generally kept open for arrivals and on a decent weather day that can mean upwards of 10 landings (I have no idea what they are transporting).

I had the pleasure of meeting one of the guys that helped design the Philadelphia Class B. The airspace is specifically designed so that all of the hospitals sit under shelves such that the helicopters can come and go without needing a bravo clearance to expedite their travel times.



Its worth mentioning that ready to fly and will fly are not the same thing and the latter is at the operators discretion. Ultimately the pilots make the go no-go decision and if the weather is dicy rescue teams generally will not fly.