Normally about 100 feet depending upon speed.
I once passed an ambulance with full lights going. It was only going 45 mph on a 55 mph road. I wanted to do 55mph so I passed it. It was hard to see ahead of the ambulance to pass because of the blinding flashers, but I made it ok. Left the ambulance in the DUST.
500 feet
All emergency vehicles in my area are labelled "Stay 500 feet behind"
Stay 500 feet behind it if it's lights are flashing (in the U.S.)
300 feet
300 feet !
500 feet.
I'm quite sure there is no emergency vehicle anywhere within which you can travel 300 feet.
The distance one must maintain behind an emergency vehicle actively engaged in a response, or a pursuit, will vary by state regulations, and even sometimes by local jurisdiction. Fire apparatus customarily has this distance stenciled on the back of the equipment. If it is a law enforcement or emergency medical vehicle you must be aware of your own jurisdictions regulations.Note: Unless you are also operating an emergency vehicle, you may NOT follow one engaged as above. It is against the law to do so in every jurisdiction I am aware of.
500 feet
the answer is approx, 300 feet.
Yes if you are just following it to see where it is going. It is however not illegal to be going in the same direction as an emergency vehicle as long as you stay 500 feet back. If the emergency vehicle is not using its emergency lights or sirens, and is not stopped at an emergency scene, you can treat it like a regular vehicle and follow it as would be legal for any other vehicle.
It is in California.