5 years if __intact__ and simply inoperative. In the event of a collision or attack the objects will break up, and the energy is not evenly distributed. Some pieces end up with lower energy and decay faster, others greater energy and thus higher orbits.
https://en.wikipedia.org/wiki/Kessler_syndrome cascades can result and even spread to higher orbits. As an extreme emergency measure it might even be wise to de-orbit everything in attempt to mitigate the damage.
Wrong. You cannot execute an orbital change with a single application of delta-v. Any pieces can at most end up in a new elliptical orbit where the high point is further up and the low point is where they started, and this subject to the same atmospheric drag there. It takes a second application of energy at the top to circularize into a new orbit.
https://en.wikipedia.org/wiki/Kessler_syndrome cascades can result and even spread to higher orbits. As an extreme emergency measure it might even be wise to de-orbit everything in attempt to mitigate the damage.