Coinbase is riddled with red tape and KYC/AML regulation, so waiting times are inevitable if you consider how much checking and validation must be done behind the scenes. They might say 12 days, but it could take a lot quicker, so they leave room for delays.
They handle a very large amount of buy orders and they are doing this with a small amount of people on the backend, so they will rather give the maximum "wait" time, to reduce the queries if the order takes a lot of time. <Imagine what would happen to their support staff, if they reported 2 days and it took 10 days.>
I would not be too worried about the amount of time that are stipulated in the notification.