when to retire a mother?

[stinkyattic]

That's the right way to keep a mother.
You can make a mother out of one of her clones just fine, and you should make one as a backup in case anything goes wrong.
Keep the old one until she is too lanky and unruly for the space she needs to inhabit, or requires too large a pot. Or you could 'bonsai' her with root pruning. There should be a sticky or FAQ entry about that.