I can't normally write in one fandom if I'm obsessed with another, so I expect GRRM will drive anything else out of my mind for a while. However, GRRM doesn't want fanfic written about his books, so at least the obsession will last only as long as I take the read the book, and won't live on for months and months, fuelled by fanfic.
But it will be a few weeks, at least. I normally read books all in one sitting - and then regret it afterwards, since I've no longer got anything left to savour. GRRM is different. The structure of his books means that I can always stop after a chapter, and eke it out, and extend it over two or more delicious weeks.
I knew this was on the horizon. I wanted to write my Walking Shadow sequel quickly, since I thought that if Howl's Moving Castle didn't get me, Serenity would, and if Serenity didn't get me, AFfC definitely would. Howl didn't... though it did prompt me to write a Diana Wynne Jones fanfic, albeit about a different one of her characters. Serenity I liked... but it didn't revive my old Firefly obsession in the way I thought it might.
But AFfC almost definitely will be the real thing. So it's probably bye-bye to fanfic for a little while, with my sequel still half finished. Oh well. It's only for a few weeks, then I'll return to writing.
(And I expect my GRRM obsession will at least relax its hold on me enough to let me write DIR drabbles...)