A neat (and presumably pretty old by the site design) article about how long satellites will last in Low Earth Orbits with varying eccentricity and perigees alongside satellites with different amounts of drag, alongside explaining the various forces that cause this orbital decay. It also highlights how setting the lower boundary of space at ~100 kms is, while not an entirely arbitrary decision with stuff like the Karman Line, where a plane would need to travel at orbital speed to achieve lift, is fairly deceptive for actual satellites, which according to the article won't last more than a few weeks below 250 kms. Wikipedia cites an estimate that Sputnik which orbited Earth at an orbit with a 215 km perigee and a 939 km apogee lasted 2 months in orbit. There are also charts of orbital decay for the ISS (Graph 1, Graph 2) and of Tiangong-1 (Graph 1, specific dates, Graph 2, Apogee and Perigee)
The following table provides a very rough guide to the lifetime of an object in a circular or near circular orbit at various altitudes.
| Satellite Altitude |
Lifetime |
| 200 km |
1 day |
| 300 km |
1 month |
| 400 km |
1 year |
| 500 km |
10 years |
| 700 km |
100 years |
| 900 km |
1000 years |
