It's best to stop shooting when they chaff.
If you accidentally have a police ship or any innocent ship targeted when you shoot it, you get an instant bounty on you, even with a single pulse from a single pulse laser. Apart from that, it's difficult to make exact rules. I did some tests that showed you could land a fair burst on a police Anaconda, and all you get is a warning.
If you do shoot the police or any innocent ship to become wanted, you must act immediately. Put 4 pips to engines and 2 to shields and start boosting away from the cops and out of the RES or wherever you are to get clear of mass-lock, then get your FSD going. It's best to jump to target another system to jump to, which prevents you from getting mass-locked by any ships chasing you. If you're not taking much damage or nobody is chasing you, you can jump into super-cruise, then plan where to go to. If you're quick, you will always get away. Every second counts.
When you get into supercruise or jump somewhere else, find the nearest low security system with a station, where you will find an interstellar factor who will clear your bounty, so that you can go back to what you were doing.
Here's an old video, that shows all that, including how to get away. Turn up the sound. Only the first 3 minutes is relevant because they changed what happens with bounties after that.
Source: https://www.youtube.com/watch?v=U3BmDRoUsKo