Welcome to FT, marcuscent. bmi miles don't expire. That said, given that many here suspect the airline may be sold at some point in the medium term, the consensus appears to be to try to spend them as you earn them. The speculation tends to focus on LH as a buyer, and my guess is that is unlikely that mileage balances would be lost in that scenario, but they would likely be devalued and cash + miles would in all likelihood disappear (although, with that kind of miles balance, why bother with the cash...).