There is no rule for this, and it's different with every child.
My girls are both bi-lingual. I speak to them in one language, and my wife (and the environment) speak to them in another.
My eldest started speaking both languages more or less together, and is happy to switch from one to the other when talking to different people.
My second would only speak my wife's language, though she understood everything I say in mine. This is starting to change just now, in the last two weeks, and she is 2 years and 4 months.
It is important to note that our languages are consistent. I always, only speak in my language, and will only read stories and show videos in my language. My wife is the same for her language.
I have noticed that after a full day with me, they will both be more likely to answer in my language, so perhaps the best answer is to spend more time with her.
In my usual Zen approach, I think that trying to force the issue is unlikely to work, as it will just create antagonism.