Season 2 should already be available by now, and Season 3, if past patterns is of any indication, should become available right after the DVD and blu-ray disks release or near the beginning of Season 4
It will premiere on Netflix 60 to 90 days after the release on DVD or Blu-Ray. Netflix will have it just before season 3 starts. Those are the terms of their agreement with AMC. Season one became available a week before season 2 started. I would expect a similar timetable for season 2.
Stuff doesn't become available that quickly on Netflix, but giving how easy and convenient Netflix is, some people wouldn't mind waiting I guess. If you really can't wait, you can consider getting them from iTunes. Every single episode of each season has been listed out nicely and in order. The episodes are in nice quality, even with HD quality options (at slightly higher price).