If you are a Blanc Pass member your reward will expire after 3 months from the time they are added to your PM Pass.
For Rose Gold members, your rewards will expire 6 months from the time they are added to your account.
For Diamond Pass members you have 18 months to spend your rewards from the time they are added to your account.
The more you spend on your way to becoming a diamond pass member, you get the perk of having a longer expiry period to save your rewards and accumulate them to spend on something special.