When is the right time to disclose significant personal info?

When I was young I endured years of abuse (emotional and sexual) from a family member. I have received counseling for this as an adult and I feel that even though it doesn't affect me now, it is obviously part of the person who I am.

I never know when, or if, to tell a person I am dating or in a relationship with for fear that it will scare them off somehow. But I want to share, get it out and not rehash it.

When is too soon, and how is it best to discuss it.
Thanks for your thoughts.