If you were charged for going over 1k miles in a particular month, but have done 12k or less at the end of month 12, you can claim a refund. This will not be actioned automatically. You will need to request the refund. That’s what I did, and it was refunded within a couple of days.
As far as I understand it, there is no time limit for Mileage Credit. It just carries on rolling over until you have used it up. The 12 months applies to where you have paid overage fees, but then gone on to do less than 12k miles in that subscription year.
This rollover is discretionary and only if you continue to have a vehicle in the same tier. If you go up a tier, you would have to start again in building up mileage credit. This is because mileage charges are higher for vehicles in higher tiers. If you come down a tier, the same applies, but I would probably argue that as I’m moving to a vehicle with lower mileage charges, I should keep my mileage credit. But that discussion probably deserves a thread of its own!