The Purple brand mattress has a life expectancy of 8 to 10 years which is in line with other mattresses.
Regardless I always replace my mattresses after 10 years anyway because I love to get a whole new bedroom set and redecorate my bedroom after 10 years anyway.
10 years is good enough for any mattress anyway and I find that after replacing my mattress after 10 years of use I sleep much better than I did on the old mattress.