If we are billed annually, do we get the whole year's credits at once?

If you pay for the year up front, yes, all twelve months of credits are issued on signing and are valid for twelve months from that date. A shorter term works the same way: six months paid up front gets all six months of credits on day one.

If you pay monthly, credits are issued monthly, because otherwise you would be holding credits you have not paid for yet. Monthly credits roll over once and then expire.