How long do you have to sue someone for defamation of character?

My ex had previously been talking about me on social media... Probably still is but I can no longer see his page. He at one point shared my image and also would make reference to me by my last name. This caused two individuals whom I don't want to get involved to recognize that he was my child's father. Comments like I'm mentally ill and he also provoked his friends to make such statements as he should take half of everything I have including my hair weave. Considering these comments are made on social media and at the time of some his page was completely open can I do something and how long do I have? I wanted to address it during my custody case but we have never gotten to it. He is not financially responsible for our child since I make so much more than him.