Not all the time
My boyfriend has a 10 year old daughter, and I would never expect her to call me mom, because no matter what happens, I will never expect her to call me mom. Her mom is a good woman and they have shared custody. She and I have a good friendship, and know that her daughter is the most important thing to all of us. She knows that I would do anything for her daughter, and I treat her as if she were my own daughter. But I would never want to replace her mom. Her mom is her mom for a reason, and you never want to take that away from a child.