March 22nd will be 500. Wrestlemania 29, on April 7th if Punk gets there with the strap would be 516.
The other chase for Punk, that no one is talking about, but should be is the list of total championship reigns. Punk sits 11th @ 403 according to Wikipedia.
It's not likely he'll catch Pedro Morales @ 1,027 but Bret Hart (627), HHH (539), Austin (529), Savage (520), and Randy Orton (420) are all within reach.
He'll pass Orton for 421 total days as WWE Champion on December 15th.
The next group is a lot harder to catch but for the sake of discussion we'll say he remains champion past Wrestlemania. If he does, 3 champions suddenly look catchable.
Randy Savage and his total overall mark of 520 would fall on April 11th 2013. If Punk survives the entire month of April 2013 as champion, he would pass Steve Austin at 529 on April 20th, and then Triple H (539) at the end of April on the 30th.
Punk would have to make it through the summer, passing 600 on June 30th 2013 going straight through till August 23rd when he will pass Hart's total mark of 654. Then the challenge of reaching Pedro Morales 1,027 can be held off till September 2014, when Punk would be able to close in on that mark shortly removed from 3 years as champion. He would pass Cena in October of 2014 if he still has the title and is about to close in on 3 years at that point.
Then we can talk about Hulk, Backland and Bruno!