It's already a mature product given it's simply an overclocked 7870 so it's lifespan will be limited. People are saying the R series, with the exception of the 290 that is a brand new chip, is a stopgap until a truly new architecture is released by AMD.
I would expect a shorter timeline of usefulness but maybe a meteor takes out AMD and Nvidia tomorrow and we are stuck in current generation for decades. Point I'm trying to make - no one cant predict which direction tech will go, but historically 1.5-2 years something is released that is twice is good as it's equivalent predecessor in the product line up.