JD - this is a question I constantly asked the one I just ended with and her response was "you will know when it is time" which I found to be an incredibly useless response.
I have read the following books:

Termination in Psychotherapy - Joyce, Piper, Klein
Positive Endings in Psychotherapy- Steven Kramer
Terminating Therapy- Davis
Good Enough Endings - Jill Salberg
Endings in Clinical Practice -Walsh

I am not sure how useful they would be - but they do exist.

Jane, I felt sad reading your post. I'm feeling the same way right now. I reached the goal I had when I went into therapy (simply, 'to stop crying'), but it seems that I "should" have more goals....why do I feel this way, because of my T's judgements about my life, some of which I agree with. I have new goals, therefore, that developed through this past year of therapy. I feel like I can't achieve them, though, so I feel like therapy is over for me. My T said she hasn't given up on me, though, so maybe we can still move forward. A friend, who is a social worker, that I emailed when I felt in such turmoil last week gave me this email response (maybe it can help you, too):

Has this therapist helped you achieve any of your goals? Did she ever help you identify what you hoped to achieve? Is your life different - even slightly better - than it was when you started? Do you journal? Can you journal with your therapist? That would give you a platform for conversation each week. I know that I have asked more questions than answered any.