For me it'd be time to end therapy once I stop learning something new about myself, when the therapist can no longer offer new helpful insights.

I should say though that knowing when to end is one thing but to actually end it is another. While I knew when to end with my ex-Ts I couldn't do it immediately. I prolonged it way past its efficiency period, which had to do with dependency on the Ts I had developed. It didn't feel good at all knowing that I had to stop but not being able to .. :-( It felt like a financial rip off and emotional exploitation..

So, while you may know when to end, it may be easier said than done..