I don't think there's any amount you have to tell right away. If you're not comfortable disclosing SI to your new T, I would wait until you are. I think it's an important issue that needs to be brought up at some point, but if you don't feel safe/comfortable jumping in right away, you can always wait until you do.

I was in the same boat when I started seeing my new T...in fact, I even straight up lied when she asked about SI, because I didn't feel comfortable enough disclosing that to her. As I got closer to her, and more comfortable, I eventually did tell her, and despite me being terrified of her response, she was very calm and reassuring about it.

So the short answer is you don't HAVE to tell anything that you don't want to. It would be beneficial to tell her as soon as you feel comfortable enough, but there's no unwritten rule that says you have to tell anything in any amount of time. Take your time with this, and be gentle with yourself.