Please remember that these are based off of rumors, so take with a little salt.
Chaos Space Marines: August
The first one is quite obvious. Chaos Space Marines are Next. We have had a lot of rumors pointing us towards August, so that should be verified in the next couple weeks. If for some reason it does not appear in August, then I am afraid that the following codex will not be until early 2013.
Dark Angels: Quarter 4 of 2012
We have the starter set appearing in September, and we know the contents to be Chaos Marines vs Dark Angels. It is quite obvious that a Dark Angels codex is on the horizon, and add to that, Harry and a couple other sources are pointing to the same Dark Angels as being after Chaos Marines.
The Question becomes are they going to be before or after the new year. I personally believe we will see this codex before the year is out towards Oct/Nov, but would not be surprised if it was early next year.
Tau: First Part of 2013
The long awaited Tau will be on many peoples want list, especially with all the allied armies that Tau can join. Even though Tau seem to get pushed back again and again, they are currently slated for the first codex of 2013, assuming of course that Dark Angels get done in Q4 of 2012.
Tau have been said to be in the first part of 2013 by several rumor sources, so it seems very solid that they are on their way.
Eldar Q3 2013
Eldar are a codex that many long time hobbyist are waiting for. I for one, cannot wait to get this codex. We are a year out on this however. Harry at least has mentioned in his rumors that Eldar are slated for Q3 of 2013, and I believe others have said the same thing, just can't think of who off the top of my head.
Mystery Codex; Between Tau and Eldar
Many people think we will see a codex between Tau and Eldar, and I think they are correct, we just don't know which one it will be. On the list of possibilities are Black Templar, Sisters of Battle, Codex Space Marines, and then it just becomes a toss up.
Here is my prophetizing........Reaching into the warp now for answers...........
2012
Chaos Space Marines: August
Dark Angels: Oct/Nov
2013
Tau: Jan/Feb
Mystery codex April/May
Eldar: August-Oct