Since the universe was 10-5 seconds old, when the hadrons formed. ;^)
The theory was worked out in the 1960's. The key features of QCD are confinement and asymptotic freedom. Confinement means that the quarks are confined to hadrons, as you mentioned. Asymptotic freedom means that as the quarks get closer and closer together, the strong-force interaction between them asymptotically approaches zero, i.e. they behave as free particles as far as the strong force is concerned. (They still interact electromagnetically, etc.)
Both of these features are exhibited by a force that is directly proportional to distance. Asymptotic freedom is obvious--the force goes to zero at zero distance--but confinement is less obvious. That arises from the fact that the quarks have finite masses. If the force is proportional to distance, you can only pull a quark and an antiquark so far apart before you put in enough energy to pry a new quark-antiquark pair out of the vacuum. The color charges cancel those of the quarks you're pulling apart, and they "hadronize": the new quark pairs up with the old antiquark, the new antiquark pairs up with the quark, and you have two "colorless" mesons instead of two quarks.
The usual pedagogical analogy is to try to obtain monopoles by pulling a bar magnet apart. It just won't work.
Maybe the Teamsters were doing a preemptive strike in anticipation of this thread. ;^)
Since the universe was 10-5 seconds old, when the hadrons formed. ;^) ========================================================
Well, if you can't be any more specific than that....
</sarcasm mode>
Your reply to PH was simply delightful; succinct and chock-full of fascinating info.
Fascinating; very similar to pulling on a spring, until you exceed the elastic limit ......