Mother plant is just a term for a plant which is used to donate cuttings for clones. For many people who have space restrictions they toss out their mother plants frequently and grow out a clone as a new mother plant.

If you need a large amount of clones and have plenty of space then you would probably keep a larger "mother plant" around and not replace it as often or ever. I've heard of people having 15 year old plants, but it's not a necessity.