One of the best places to look for that type of info is at
www.alea.org (airborne law enforcement org).
That being said, the requirements vary a fair amount depending on what side of the country.
Most require that you are a current police officer. The understanding for that is you know what is happening on the ground and what your partner is doing.
I have heard of some training pilots from scratch, but that is very rare. Most require you to have some kind of license. LAPD requires a fixed wing pilot license and then they will start you from scratch on the helicopter. LASD requires a commercial helicopter license and then gives you around 80-90 hours of flight training plus ground stuff. Both require you be a ground officer several years, then an observer for several years before you can work into the pilot slot.
Prior flight time, other then what is required for the license, does not make a big difference at these departments. They will give the time required to bring you up to what they want.
Most departments pay the pilots several pay grades above the ground guys.