I went for two years. For a year before vocal surgery and for a year after. Was completely worth it. My insurance covered it, too. I highly recommend it, and my voice surgeon (Haben) required it. Make sure they are a speech language pathologist. I made the mistake of going to a woman that was a licensed clinical social worker that worked with trans clients, but also thought she could do vocal feminization therapy. When I told my speech pathologist what exercises the lcsw was having me do, she was aghast.
Also, speech therapy for feminization is not about learning to speak in falsetto. It deals with:
Prosody, resonance, intonation, inflection, frontal focus, word choice, and articulation. Pitch is only part of it.
Here is a good resource to get you thinking about the proper things to work on:
http://www.nyspeechandvoicelab.net/transgender/voice-feminization/