You generally bid for one calendar month (some airlines use 4-week bid periods). Your bid must usually be submitted around the middle of the previous month, and you find out the results around the 20th.

There are two systems:

1) Hard Line: The company constructs schedules (called lines), and organizes them by quality, ie the number one line will probably have 18 days off, weekends off, short trips with good layovers in SAN, SFO, SEA, etc. The number 390 line will have 10 days off (Tuesdays & Thursdays), crappy trips with redeye flying and short layovers in places like DTW, TUS, BFL. Everybody submits a preference for which lines they want in priority order. The number one guy in domicile simply bids one line, and he gets it. The number two guy only has to bid for two lines, etc. The number 290 guy will probably bid the bottom 100 lines, cuz he knows he will never get anything higher than that anyway.

2) PBS: This is a computer program. It has a lot of options, and you tell it what you want (days off, credit, layovers, etc). It then will attempt to build you a custom line using trips that company has already built...the senior guys get dibs on the best trips, but PBS will usually give you at least some of what you want even if you are junior. It is NOT going to give a junior guy a 30 hour layover in SAN, but it will give you your wife's b-day off. Some folks complain about PBS since it gives the senior guys EVERYTHING they want, but it has always given me the days off I need, and I care a lot more about that then where I layover when I'm working. All airlines will be using this system in the near future anyway.