How quickly it "starts to work" is entirely dependent on a) How honest you are, b) how open you are to outside perspectives, and c) how much baggage you've got to unpack.
More caveats to this...other things I've found...
- If you don't know what specific issue you need to work on, it may not even "work" at all. You'll need to introspect and research and /or even seek others' input if you don't know already.
- A lot of therapist I've come across, especially in online organizations, seem keen on using unstructured talk therapy. It can be great for basic validation, but not if you have more significant and complex issues. If you know your specific issue, it might help to research what kind of therapy modality works best for it. Then, look for therapists who use said modality and who have treated said condition.
I learned these things the hard way.
Well...if you have anxiety, CBT is supposed to be the primary therapy modality for that lol.
What specific issue are you looking to address? If you figure that out, you can find out which therapy modalities are shown to work best for it.