Qmiles will not expire as long as member does Qmiles accrual / redemption activity with Qatar Airways or partners (Air, Non-air ,).
By default, if members have no activity for 36 consecutive months, then all of their balance QMiles will expire. Expiry will be three years from their last accrual or redemption activity.
For Example, If the member has done last accrual/redemption activity on 2nd January 2018 and has not done any accrual/redemption activity after that, Total balance of Qmiles will expire on 01January 2021
Platinum members Qmiles will not expire as long as they retain the Platinum tier at the time of each Qmiles expiry period