Hi Jayelleseven,
The airlines work on something called the "seniority list".
As an airline pilot your life is goverened by such a list. It'll rule your life as well as your familes.
When you join the airline your place on the list is right at the bottom. So if they already 400 pilots when you join, your number would be 401.
As other pilots higher up the list than yourself, leave the company, retire, etc then you will climb up the ladder to a more senior number.
If the airline employs another batch of pilots after yourself then your seniority number will be higher than theirs.
Being higher up the seniority list has a few perks. If your company have to make pilots redundant, guess who they'll dispose first? Yes those at the bottom of the list. You can bid for the routes that you'd like to fly, also bid to be based at another hub/airport, etc etc.
Anyway as you become more senior within your company then so the chances of your number coming up and being made a Captain (as long as you pass your captain course).
Not sure of the majors over in the states but here in the UK British Airways say something like 12-18 years from ENTERING the company to becoming a Captain. Some airlines will recruit captains directly from outside BUT you would be expected to have a good few thousand hours as a captain on their type of equipment already.
Another airline, this time a cargo one, Atlantic Airlines, is a little smaller and they say roughly 2 years in the company will see you move from the right hand seat to the left.
At the end of the day it all depends on which company you work for, your experience, some cases which fleet your on and the dreaded seniority number!!!!
In the UK the legal minimum amount of flight time to become a captain of a jetliner is 1,500 total time, however this is not normal - pilots will normally have thousands and thousands of hours before gaining their captaincy. This is because you have to have a JAA ATPL unfrozen. This also dictates a minimum of 500 hours Multi Crew time on multi crew aeroplanes (namely airliners).
Just a disclaimer: I'm not an airline pilot. I'm en-route to becoming a professionally qualified pilot.
Hope this helps.
Best wishes,
Charlie Zulu.