What a time to live in, when papers from 2015 are "really old" in 2017.
It highly depends on the field you're in though.
1 deep learning year is about 49 dog years.
What is "exponential growth"!
It's more logarithmic, isn't it? Right now we're at the beginning stage, where there's massive discoveries and changes happening, but in say, 50 years time, there won't be much changing year-to-year.