As it stands, there are some rumors as to how the following seasons will go (this will contain minor minor spoilers, just forewarning):
Game of Thrones: 10 episode season
Clash of Kings: 12-13 episode season (as it's a bit longer than the first)
Storm of Swords seems to be posing an issue, as the book a) is jam-packed with content. There are just tons of events that are exceedingly important and b) is extremely long, much more so than the other two, they're considering the idea that it may need to be made into two seasons.
Feast for Crows: This is a bit of a tricky one as well because the way it was written, it follows about half the characters from the first 3 books, selecting the ones to follow more by their geographic region than anything else. The plan is that Dance with Dragons will cover the ones not covered in AFFC. In order to not throw a wrench into the series by taking away key characters and breaking the flow of the story (as we don't have "POV" chapters in the show), they would combine AFFC and ADWD. This could conceivably be 2 seasons as well, calling one "FEAST FOR CROWS" and one "DANCE WITH DRAGONS", but realistically they would contain elements from both books. As for the final two in the "heptalogy", there has been little mention of this because we're all still eagerly awaiting book 5. That being said, it sounds as if a good chunk of the content from Dance was bumped to book 6, which means 2 things:
1: Book 6 is currently being, somewhat passively, worked on
2: Book 5 is super close (or so it seems)
THIS IS EXCITING STUFF! WOOHOO! (hope this was helpful!)