It depends on the company, but I'll tell you what I'm doing right now (and hopefully for the last month! I'll find out tomorrow when our phase II bid packets come out).
I commute to reserve from Salt Lake City, UT to Newark, NJ. Because of the distance of the commute and the few number of flights per day between these airports, I generally have to leave Salt Lake at 10:00 a.m. the day BEFORE reserve starts to make sure I'm in Newark. But the why is more important.
Our commuter clause says (basically) we need to have two flights to get us to our base that, 24 hours before the flight departs, have seats available that will get us in 30 minutes before our flight or reserve time starts. Because there are only two flights a day between Salt Lake and Newark I generally have to leave at 10:00 a.m. the day before to make sure that I have my primary and secondary flight to count towards my commuter clause. There are some exceptions, like today, where Continental also runs a red eye, allowing me to use the second Delta flight at 5:00 p.m. as my primary and the Continental flight as the secondary.
Now here's another example: let's say I'm based at LAX instead of Newark. That commute has A TON more flights, so I could possibly leave the morning my reserve would start if I have two flights that get me into LAX 30 minutes before my reserve period starts.
The chances of getting on a flight depends on how many seats there are, the weather, what phase the moon is in and what other commuters are trying to do the same thing and how you stack up against them seniority wise.
I'm currently flying about half of the time that I'm on reserve. On my last block of reserve days I sat for 6 days until I tried to go home, at which point scheduling gave me a turn up to CYUL. I'm expecting to do a little bit more on this next block.
The length of time you're on reserve depends on a bunch of different factors. It depends on your base, your aircraft, how many people are hired after you, how many people are leaving your base, how many people are upgrading, etc. etc. You could be on reserve 0 months to 12+. I think Ready2Fly spent a year and a half or more on reserve with Eagle. It looks like I'll spend 7 months on reserve (that number is based on date of hire) in Newark before holding a relief line, and then maybe I'll hold a hard line this summer (so about a year).
