Well, it has already been announced that book 3 will be split into two seasons (although I think only the first half has been greenlit). The producers already know the rough storyline of the last two books, given that Martin isn't the youngest and, well, rather obese. If the series lasts until the end, I am sure they'll figure something out. And, of course, there is no obligation to film a season each year - the Sopranos needed 8.5 years to get through its six seasons.
HBO and GRRM collaborated extensively to simultaneously promote Game of Thrones (TV) as a promotion of Dance with Dragons, which was released the following year. Storm of Swords topped out at just shy of 1,000 pages and HBO is splitting it into two parts, as the paperback was in Europe. DwD was even longer and may be treated the same way. So, if we assume that GRRM and HBO continue to collaborate in the same manner as they have been, that GRRM sticks to his current writing projection, and he is not (in fact) Robert Jordan, then an optimal publication/production synergy would follow...
2011 Game of Thrones, season 1 (
GoT);
A Dance with Dragons published
2012 Game of Thrones, season 2 (
CoK)
2013 Game of Thrones, season 3 (
SoS p1)
2014 Game of Thrones, season 4 (
SoS p2)
2015 Game of Thrones, season 5 (
FfC);
Winds of Winter published
2016 Game of Thrones, season 6 (
DwD p1)
2017 Game of Thrones, season 7 (
DwD p2)
2018 Game of Thrones, season 8 (
WoW);
A Dream of Spring published
2019 Game of Thrones, season 9 (
aDoS)
I don't think this is an unreasonable expectation, but these assumptions are big ones. It's very likely that
aDoS will be a hefty manuscript. I'm also not holding my breath that HBO will continue to produce this show for another 7 years. They're profiting now, but this wave of post-Whedon fantasy/sci-fi enthusiasm is just a fad. Soon enough the public will lose interest in all this stuff and we'll just have to settle for reading it, as we always have.