Maybe, just maybe this belongs in MPSIMS. However,
asuming we never launch another rocket, how long would it take for all of the existing debris we have put into space to return to Earth?
[Moderator Hat: ON]
No, it doesn’t belong in MPSIMS.
Nor does it belong in Great Debates.
It belongs in General Questions. And that’s where I’m sending it.
David B, SDMB Great Debates Moderator
[Moderator Hat: OFF]
Well, according to Russell Hoffman, http://www.animatedsoftware.com/spacedeb/spacedeb.htm
it would take “thousands and even millions of years.”
I’ll let you putter around at NASA. Enjoy! 
http://sn-callisto.jsc.nasa.gov/
Depends entirely on what orbit the junk happens to be in. A bunch of it ain’t ever coming back, and other pieces are probably reentering the atmosphere as I type.
And then, for your added edification, there’s The JPL in Pasadena.
Cool site, more info.
Cartooniverse