Bisexuality Support Group
Bisexuality is a sexual orientation which refers to the aesthetic, romantic, and/or sexual attraction of individuals to other individuals of both their own and the opposite gender or sex. Most bisexuals are not equally attracted to men and women, and may even shift between states of finding either sex exclusively attractive over the course of time.
I don't know how old your mom is but she probably grew up in a time when this was supposed to be not talked about or kept secret. That time has passed.
No matter when you come out on social media, she will process however she will. and it has nothing to do with you.
Since you didn't mention any negative reactions, it may be that many of the people in your life already knew or don't really care either way. Which is great, because they shouldn't. You're still their daughter, sister, niece, etc. and you are still the same person they love.
So what is a reasonable amount of time for YOU to tell the world? That's what matters.
Tell your mom that that is what you are going to do if you like. But don't let her decide for you. If people ask your mother about it and ask, "Is your daughter bisexual because she put it on facebook?" She should say, "Yes." End of discussion. If they ask anything they shouldn't ask, she needs to say, "She's my daughter. She the best thing that ever happened to me and I don't care who she's with as long as she's happy."