depends alot on where you live, your car, and how smart you are. lots of towns dont allow cops to chase, so once you have a somewhat good lead on the cops they will give up, but they might go around looking for you, cause they dont want somebody going like 100 through neighborhoods running from a cop who is also doing 100.
i was on a windy road doing 30 over, and a cop past me going the other way, hit his brakes and turned his lights on, i knew he was gonna have to do a 3 point turn around so i sped up, and i knew by the time he was back up to my speed id have like 30 seconds on him atleast, and knowing my car is probably a bit faster, and handles alot better, i was only gonna be gaining, so i ran and i didnt even see the cop behind me once. if your on the highway, and its buisy, if you can take an exit within like 1/2 mile or so do it and drive fast to a parking lot or something.
most of all its good judgement, sometimes you can run without trying and get away, other times you will get caught almost nomatter what.