How long is the orbit of an orbiting satellite just before it falls back to Earth.?

The Earth's magnetic field can go well over 300,000 miles out into space, but most of the time space probes start to fall around the Earth when it gets to be around 360 miles up. At 25,000 miles per hour, it should take about 11.9856 hours to reach the point where it just starts falling to Earth. Once it does this, it should take about 0.0144 hours or 0.864 minutes or about 51.84 seconds for it to fall back to Earth and maybe a little slower than that because it has to go about several hundred miles per hour once it reached about 20 miles above the Earth. Hope I helped!

Maxwell

How long is the orbit? Starting when?

Lets take the case of Mir. The deorbit was done in stages, starting at 00:32 UTC. The last stage started at 5:08 UTC with the final retro-rocket burn. Mir hit the atmosphere (100 km) 36 minutes later at 5:44. Within 15 minutes it was burned up, with some debris falling into the South Pacific.

Sky Lab didn't have retro-rockets, but the controllers could aim it. They sent it into an end-over-end tumble at 3:45 a.m. EDT, and it impacted land (near Perth Australia) 8 hours 52 minutes later, at 12:37 p.m. EDT. I couldn't find out when it hit the atmosphere.

Depends on the altitude, if everything remains constant.. it'll be there forever. If it hits some debris/outside force, might get knocked off orbit. If it stays there for too long, the materials might decay.. depends on which material it is.. the ones with lower altitude will decay quicker.

Sorry Idk but he means- How long was the last complete orbit?

Just felt like I had to clarify.