How remove the turbo tax annual audit defense coverage if I have already paid for it? How long does the period of membership last?

If you buy audit defense when you check out, that covers that year’s return only, for as long as that return might be audited, usually three years. If you bought audit defense in 2018, that covers your 28 return. If you want your 2019 return to be covered, you have to buy it again for your 2019 return, and so on.