which phrase really exists?
(Situation between two persons)
Question: how far does a mother's love can be?
Answer: more than you can imagine
more than you could have ever imagined
Do these two exist or just one?
Both your bold expressions are grammatically correct. Their meanings are similar, but not the same (the difference is the tense)
The part which is poor English is the question "how far does a mother's love can be?" The auxiliary "does" is not used with the modal "can". And you probably want to use a more specific verb than "be". Perhaps "How far can a mother's love stretch?"