Product review (1 stars): They will probably last just long enough to get past the refund period.
I bought these in a 4 pack (no longer available), and although the first one worked fine and lasted almost a year, the second one I installed only lasted a few months, and the third one didn't work at all.  I contacted OnePurify and they said they don't honor any refunds after 90 days.  That nice and all, but 90 days isn't typically long enough to know whether a product that is supposed to last 1 year is defective or not (especially when bought in a pack).