I have such a bad concept of what's "too much." It's strange though because all my depth of thought centers around one or two narrow areas... so if they don't come up, I'm not open at all, but if they do, the person strikes an acid mine of things I almost definitely shouldn't have said.
In theory, I really think we should (and I try to be) as open and honest as possible. If conversation hits the "deep part" filled with the things I should probably never say, and the person gets scared and runs (or tries to hurt me with it), I can at least console myself with the fact that "well, they asked... so I didn't really do anything wrong." I've learned recently to preface it with "this answer isn't what you think it'll be... are you sure you want me to answer that?" though. You might get hurt... in fact you almost definitely will sometimes... but on the other hand, you might just strike gold. I personally think the relationships you build without reserving parts of your thoughts and feelings are well worth the trouble that other ones cause... at least for me. Plus, people tend to respond very well when you're wide open about weakness in yourself.
Actually I'm not sure that's recommend that for INFJs though, now that I think about it. I know now that I won't hurt anybody if they hurt me (er... with practice and a few bad mistakes), so I only have damage to myself to consider... and that's not worth that much to me. If I were more prone to lash back at the people who hurt me, though, I would have to rethink that.