Theoretically, there is some crazy grower out there who has been harboring the same mother plant for 20 years or more. If that person took the right care and only so many cuts per year, it could keep going. But, eventually, the mother will wear down and you might not be creating healthy clones. At least, not as healthy as they could be. Instead, you should change out your mother plants on a yearly basis. Let the old mothers finish their grow cycle on a 12/12 or toss them out due to depletion. Create a new mother from the best clone available.