We have a mandate to bring those back, or at least we have procedures and protocols in place to ensure that they will come back and not generate space junk. A lot of low-earth orbit spacecraft do come back just because of their relativity to earth. Being closer, the gravity force is stronger on them, so they do come back over time. Usually it's somewhere between five and 10 years. It could be a bit longer.
Some spacecraft have forced de-orbiting mechanisms in place so that you can bring them back immediately. A lot of newer missions, especially going out of the U.S., Canada and Europe, do have very robust plans in place to ensure that we don't create more space junk.