When do you start using the new female or male pronouns to refer to a transgender person?

I have a very close friend who is transgender, and I began using female pronouns when referring to her as soon as she told me she was going to transition to be female. Technically, she’s still living with male parts, but in her mind, she’s a fully functioning woman.

What do you think? If gender is between the ears and sex is between the legs, then when should we start changing pronouns?