The counter is not reset every year ( but may be worth checking with amex). You are only allowed to earn one voucher per year. If you spend 15 k in 6 months, you get your voucher, and still earn ba miles on spend in the following 6 months. You do not start earning towards the next year until you reach you membership start date. ( If you phone amex they will tell you the exact date).

As far as I know you can spend as long as you like to get to the target spend.