As I see it, even an optimistic assessment of the hopes that GRRM will complete the series will have him finishing in AT LEAST eight books, if not more.
A satisfactory finish in 7 books is not theoretically impossible, but that would require a radical switch in GRRM's writing style and storytelling efficiency. Such a radical change is about as unlikely, given GRRM's history, as the odds that he will suddently start cranking out one 1000-page volume every six months.
I consider GRRM finishing the series in 15 years to be "optimistic". Any optimistic assessment requires him to do some combination of the following: (a) start churning them out faster; and/or ( b ) provide more story per volume.
If GRRM finishes in 2 more volumes, that gives him a perfectly doable 7.5 years for each 1000 page volume. But, given his history, I just cannot see GRRM providing enough plot-progress per volume to finish in 2 more books. Perhaps another writer could do it, but not GRRM.
If GRRM finishes in 3 more volumes, that also requires him to radically (but less implausibly) improve his storytelling efficiency; and gives him 5 years to write each 1000 page volume.
If GRRM finishes in 4 more volumes, then I suspect he must still improve his storytelling efficiency. But he must also churn out the volumes faster than he has been doing, giving him 3.75 years for each 1000 page volume.
Etc. etc. etc.
Of course, we can also redefine the above definition of "optimistic" by giving him 20, 25, 30 years, or more to complete the series.
Edited by Fearsome Fred, 06 May 2012 - 05:21 PM.