There are a couple different ways to figure out the velocity of a cars speed; the most common ways are to pace you or the use of a radar gun or laser gun. A radar gun sends out a frequency that slowly gets larger as it moves away from its origin meaning, the further you are from the origin point of the radar gun pointing at you the less accurate it will be. The radar gun might be picking up multiple cars at once until the vehicles are close. Due to a radar guns inaccuracy at longer distances laser guns are preferred in certain areas. A laser gun sends a small focused signal directly towards its target. You can hit a car accurately from over a mile away if aimed perfectly. If you own a laser detector they can pick this beam up from anywhere between one and three miles away.
Radar detectors aren't near as sensitive but do not need to be either as a radar gun needs to be much closer to zero in on one vehicle. How much time you have completely depends upon the brand and model you buy. If you are the type of person that just plain wants total protection and awareness and has the money to spend look into the Cobra XRS 9960G, the Cobra XRS-R10G, and the Escort 9500IX. If you're just looking to be safe the majority of the time but don't want to spend the big bucks both Cobra and Whistler have many models that will suite your needs.