Simply one year isnt too correct.
In my experience, it is the next upcoming miles expiration date (end of a month).
For example,
Your annual mileage expiration month is July?so your next upcoming miles expiration date is 31 July 2018.
a) if you refund your miles today (13/14 Jan 2018) , the miles will expire on 31 July 2018.
b) if you refund your miles on 1 July 2018, you whole amount will still expire on 31 July 2018.