At the US airlines, everything, and I mean everything, is based on seniority.
Whenever the airline decides it needs more people in a certain category (say Airbus Captains & F/O's because they are getting more planes), then they hold a 'vacancy bid'. What happens there is that every pilot chooses what airplane, base, and seat they want starting with the most senior pilot, and then working their way down the list, with the bottom person (least senior) getting whatever is left over.
Working with that kind of vacancy bid, your decision on what type of plane to fly or what seat to bid for may have a lot of variables, or none at all. If you are a brand new hire, you are told what and where you'll be flying. By the time you've been there a while you might have a choice - "do I want to be a junior F/O on a large plane, or a more senior one on a small plane?". The larger plane may pay more, but your lifestyle suffers because you are junior and have to be on reserve (on call) while on the smaller plane where you are more senior you can hold a regular schedule and know what days off you'll have for the month. The same thing come up when you have enough seniority to bid for a captain's slot. I know a lot of F/O's who stay in the right seat because they are based at home and can bid for exactly the schedule they want. They may also be able to be based in their home town. If they were to become a captain they would be junior again, and have to sit reserve, possibly in another city which would add the time and expense of commuting, or even a move. The decision on how long you spend as an F/O (once you are able to bid to be a captain) is entirely up to the pilot. Before that point however, there are a lot of other variables.
The length of time you spend as an F/O is very much dependent on how fast the company is growing. If the airline is healthy and growing (i.e. Southwest, AWA, Jetblue) then it might be only a few years as F/O. However I know of people at the bottom of USAirways list (of those still flying) that have been there 15+ years and are a very long way from upgrading, because the company is shrinking. Some of them had been Captains before, but now because of the reduction in size at US, they're back to being junior F/O's. It all depends on how the company is doing. At some of the smaller but growing commuter airlines, I've seen upgrades from F/O to Captain in less than a year, and as I just mentioned, there are USAirways F/O's on furlough that have well over a decade with that company. You pick your airline, and take your chances.
In the end you can certainly make a rough guess based on current growth and hiring at your airline, but as we all found out on 9/11, those guesses can become moot in a blink of an eye.
One smooth landing is skill. Two in a row is luck. Three in a row and someone is lying.