Hiya Flyboy,
As other have pointed out, the best answer is "it depends".
At our shop (NWA), we don't have a "forced" upgrade policy, and so people who wish to maximize QOL (quality of life) tend to sit on one position when they are very senior despite the money. We had one guy who had his own money sit on the 727 FE seat for like 10 years. He could have easily held DC-9 or 320 captain. But he was #1, and with our computer bidding system, he basically named his trips and schedule every month.
That was obviously an example of the extreme, so let me give you my personal example. I sat FE on the 727 for about 2.5 years. I had flown for the regionals for 7 years, and so I decided to concentrate on QOL a bit. Right after the 1 year point (just after probation ends), tons of 727 FEs bailed to other equipment for higher pay (and/or a flying seat, although lots went to DC10 or 747 FE), so I went from bidding from 35 out of 75 to bidding 6 out of 75 fairly rapidly. By the 1.5 year point, I was naming my trips (day trips, nothing reporting before noon). I never moved up past the #5 spot because the remaining people were senior, but were doing the same thing I was.
Friends on the DC-9 reported that they were basically getting what they wanted by around the 1.5 year point. Of course, this was in a period of upward movement.
Once you change to a more senior piece of equipment, the cycle begins again. It also depends on the "character" of the base and what kind of trips you like. At NWA, in DTW just about everyone commutes (%85+) so multiday trips tend to go senior, whereas in MSP, day trips go senior (but only the morning reports, because people want to be home when their kids get home). Also, some bases go senior to others, so you might move upward faster in a less desirable base.
Hope this helps...
Richman