How often should I open up?

And writing things out just for yourself can really help when it comes to things you can't bring yourself to talk to anyone about. You don't have to show it to anyone, and you also always can later choose to show it to someone trustworthy.

That's a really good question and I think unfortunately that it always depends on a lot of different factors. You should have at least some people in your life who you can tell about everything, but that doesn't mean they'll be emotionally available for you to tell about your feelings all the time. That's where therapists are most useful in my opinion, ideally you should build a relationship with a trained therapist where you feel safe enough with them to be able to talk about all of your thoughts and feelings with them during sessions. Between therapy sessions or during times where I don't have any loves ones available who have the mental/emotional bandwidth to handle me venting to them at the moment, I try to journal. It's hard to get into the habit of doing, but it really helps when I do manage to get myself to do it.

As often as you want with people who are willing to listen. Always ask before telling people stuff cause it might trigger them.

As much as you want