I have written my last therapist once a month for 4 years now. However, she always does do small replies. Another difference is that she is my backup therapist now. I try to make a session with her once a year, but she's too expensive for me now, so I don't know when I'll be able to see her next.

I did try to keep in contact with my past pdoc. I would write her once a year. But due to circumstances, I will never get a reply anymore. So I have stopped writing her.

Do what feels right for you. If it helps you, then write. If it feels like a chore or too difficult because she doesn't reply, then don't write. And even if you choose not to write, you can always write in the future.