We have to remember there are very, very few examples of big cable (regular or premium) shows that last 7+ seasons. The first season of GoT was quite successful, and DVD sales should be impressive; I'd imagine S2 will debut with even bigger numbers. But that's no guarantee for future seasons, especially given the complexities of later books. Is ASOS even able to be done in one season, for instance? How will actor contracts, budgets, and children actors growing up effect the show in the future, etc? So I wouldn't assume HBO's pressure will have a long term impact on Martin.
That being said, for the purpose of discussion I'll assume the show explodes in popularity and challenges True Blood as HBO's flagship series; I'm going to assume the expensive Boardwalk Empire fizzles out. There is already talk of ASOS being split into two seasons. I don't know if it would be possible to shoot them both at the same time, given weather conditions and budget constraints; HBO could also just allow 14-16 hours for one season. But even after ASOS is finished, there's no way AFFC will be used as one season; it will be mixed with ADWD, and I'd imagine much of it will be cut.
ACOK=2012
ASOS (15-16 episodes?)=2013
ADWD/AFFC Part 1=2014
ADWD/AFFC Part 2=2015
Martin has said he will start working on TWOW next year, and already has around 100 pages completed. Assuming HBO continues to premier the shows in April and end them in late June, Martin will have from Jan 2012 to early/mid 2015 to finish TWOW. I'm assuming he'll be providing Weiss/Benoiff with the book early as he did with ADWD.
If ASOS is split into two seasons that would give him one more year to finish, but I doubt HBO would allow them to split it and ADWD/AFFC.