When was your last level-mileage earning transaction?

Add two years to the year of your last such transaction; leave the month as it is; move the date to the last day of that calendar month.

That is the date that ALL of your Award Miles will expire (including any "younger", non-flight Award Miles earned after the date of that final level-mileage-earning flight, from shopping, car hire, hotel stays, credit card transfers, mileage purchase, transfers or gifts from third parties, cancellation of previously-booked award tickets, etc).