What's the meaning of this sentence?
How a long time you have been mother!
How long have you been a mother?
(Or "How old is your oldest child?")
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.
Or, "What a long time you have been, mother!"
Or, "You've been a mother for a long time!"