It makes sense that as long as the TV series remains popular and assuming the number of seasons equals the number of volumes, and since the TV series started in 2011 then the final season will be 2018. So Martin probably has to finish the 7th volume and presumably publish it in 2017 (soon enough that he doesn't hinder shooting the final season).
1. Game of Thrones (1996)
2. Clash of Kings (1999) 3 years
3. Storm of Swords (2000) 1 year
4. Feast for Crows (2005) 5 years
5. Dance with Dragons (2011) 6 years
6. Winds of Winter (2014?) 3 years?
7. Dream of Spring (2017?) 3 years?
It looks like he lost a little momentum for #4 and #5 but I think we can count on the pressure of TV production to motivate him to finish #6 and #7 by 2017.
OTOH factors may conspire to extend the series to 8 novels. Nobody wants the golden goose dead.