I think around 9 to 10 months...
my first born said pretty much everything by 12 months - like cars, elephant, tree, dog, cat, etc - EXCEPT FOR MOM!!!!!!!!
I would point at ME and tell her say "mom" she wouldn't! She just gave me a blank stare! So one day after couple of more tries - I said, "say (myfullname)"... and SHE DID!!!!!!!!!!! LOL
She felt it was odd because everyone else around me would call me by my name... so she did not want to say "mom" either! =)
Check with the hospital where u delivered. Maybe there was a switch.