When I started to do cry it out, the doctors told me the average child gives up after 3-4 days. Even with a consistent routine of soothing, stories, bath, etc, my little boy cried every night for a month. Some nights it was 45 mins other nights 5. As I discovered, he is extremely persistent and very sensitive to schedule changes (he likes to eat on a schedule too).
Any deviation from his schedule, going out of town or skipping 1 of 3 naps (until 9 mos, then 2 until 18 mos) for grocery shopping for instance, or staying up late one night, would result in starting over with the sleep training. But I kept with it and now at almost 2, he usually says "taku" (thank you) and "g'tie" (goodnight) as I leave the room. He rarely cries anymore, but we do still have the occasional nap where he cries for an entire hour without going to sleep. Going out of town or other disruptions are not so much of a problem--he snaps back into his schedule right away now.
So to answer your question about how long it will take: it depends on your son's temperament. If he is like mine it could take a while, but most likely it will take less time. I also recommend Weissbluth, it is one of the things I give new mothers. As you wait for your child to stop crying every night, know that you are doing the right thing. A well rested child is a joy to have around, learns faster, and even in the "difficult" stages is easier and more fun.