Well, I heard by from someone, but I stand corrected, nasa [0] says:

  Debris left in orbits below 370 miles (600 km) normally fall back to Earth
  within several years. At altitudes of 500 miles (800 km), the time for orbital
  decay is often measured in decades. Above 620 miles (1,000 km), orbital debris
  normally will continue circling Earth for a century or more.

[0] https://www.nasa.gov/news/debris_faq.html