As to Wikipedia, it is planned to last about 8 seasons and it's unlikely that it will stretch to much more than that.
Showrunners David Benioff and D. B. Weiss [...] envision the series to have a scope of some 80 hours, about eight seasons' worth of material.
As to the particular adaptation schedule, the previous seasons pretty much aligned with the books, with the 1st season featuring A Game of Thrones, the 2nd A Clash of Kings and the 3rd and 4th the 1st and 2nd parts of A Storm of Swords each (with some parts of the later books). The next seasons will likely feature a more mixed adaptation, interspersing parts from A Feast for Crows and A Dance with Dragons to equal degrees.
Seasons 1 and 2 each adapted one novel. For the later seasons, the creators conceive of Game of Thrones as an adaptation of A Song of Ice and Fire as a whole, rather than of individual novels. This gives them the liberty to move events back and forth across books according to the requirements of the screen adaptation.
There is also this interesting table which gives a very detailed adaptation schedule, comparing episodes (left) to book chapters (top), for the first 4 seasons (and also shows the gradually less linear adaptation towards later seasons):
This is largely due to the fact, that as much as I know (though, I admittedly haven't read any of the books), A Feast for Crows shows the events in Westeros and A Dance with Dragons the events in Essos at the same time, while for the TV show it is more entertaining, established and contributory to the attention of the audience to intersperse those storylines with each other. Add to this, that the next books aren't published yet and the TV series might very well end before the book series has been finished, which will likely lead to a "rougher" adaptation than having a prewritten book series, even though George R. R. Martin has large story-wise input to the TV-show.
...Benioff and Weiss said they do not want to pad Game of Thrones out so as to wait for George R. R. Martin [...] to finish writing the last two novels. Knowing the broad outlines of Martin's intended ending for A Song of Ice and Fire, and concerned that extending Game of Thrones to ten seasons would kill its sense of momentum, they consider it possible (but not preferable) that the TV series ends before the last novel is published.