Martin must be spending a lot of his HBO money on homegrown pharmaceuticals, because he is out of his gourd if he does not think he is going to be flattened by Thomas the Tank Engine before he is even halfway through writing the next book. Once Book 3 is finished in the first third of season four, Books 4 and 5 will be expended by the end of season five. I suggest you familiarize yourself with the name Brandon Sanderson.
Once the story really splits after all of book 3, they're going to have a ton of ground to cover as everyone gets spread out. It's gonna slow the overall telling down quite a bit. The problem is gonna come after that, presuming book 6 comes out in the next two years.
Assuming book 6 would be seasons 7 and 8, that only leaves him about 5 years to complete book 7.