Illifer, AGoT came out in August, 1996 (all facts herein taken directly from Wikipedia). That is fourteen years ago, and we had four books in our hands in 2005. That is an average of 2.25 years per book, not counting the writing period for the first one. However, add the five years since AFFC, and you have an average span of 4.25 years per book, again, not including the writing period for the first one. Admittedly, ACoK came out "quickly" (at least by GRRM's average standard), in November 1998, because according to Wert's info, it was largely written with AGoT. ASoS came out in August of 2000. AFFC almost five years later.
So, the first three came out fast, at least in comparison with AFFC, which took five years from ASoS. ADWD appears to be on the same track, albeit slightly longer. So, taken from the 1991 writing point, and assuming a release date in 2010, five books in 19 years. 3.8 years per book, on average, though it is the last two that really skew the whole thing, as they account for more than half of the writing (or not writing) time.
I think some of the criticisms of AFFC, in that it expanded the detail of the story far too much, are fair, and have led to GRRM's current problems, naming solving the Meereenese knot without needing five other novels to resolve the plot lines. Authors have a tough job. Their stories are not immutable, and if they make an error of judgment (not a mistake) in how a particular part of the story is written, then it can take a very long time to deal with that part of the story, unless one employs a ham-fisted and unliterary device like a sudden death of a major character (actually, Brandon Sanderson got rid of a minor, annoying plot line and character at the beginning of The Gathering Storm, in such a manner (it worked very well), but this kind of device won't work for a major character when there are major unresolved plot lines (imagine asking yourself "he's dead? Then why did I read four novels about him? and he didn't finish the thing he was supposed to!). I think that is what GRRM is struggling with now. He doesn't want to expand the story (that's my guess, anyway), and after AFFC, is under fan and personal pressure to tighten the story considerably.
I don't think anybody, least of all GRRM, wants ADWD to continue to expand the story. His difficulty now is resolving his current plot issues in a manner that tightens the story and tips it towards resolving the plots in the upcoming 2 books beyond ADWD, and does so in the style and literary ability which we all expect from him. Not an easy task.