Longer than some, not as long as others. COS is different because the top three quarters never leave, it's a small base (and appears to be shrinking) that is difficult to commute to since MX base trips usually start very early and end very late. And most people there live there so have no motivation to transfer out. DEN has a lot more movement in and out, which can be good and bad, as you will move up and back as people come and go.
DEN reserve FO's have been here at least 2 years, but there were no upgrade classes run for about 6 months, and no one was leaving for other airlines so that stifled movement a lot. All of that has changed, so for a new hire, I would plan on 2 years, and you will be pleasantly surprised when it turns out to be quite a bit less.