I don't think there's a set definition, but the idea is that you're presenting your target gender, whatever that may be, full-time. Meaning, you don't deliberately go back to presenting your assigned gender for work, to do business, to visit your family, etc. If you've changed your name and pronouns, and use the new name and pronouns exclusively, and present yourself in a way that's congruent with the change, that's good enough. Where it gets tricky is for the gender-fluid folks who may not have a set name or pronouns they prefer. In that case, I'd think a lot would depend on how open-minded the doctor was, and the content of the therapist's letter (if it's needed). The system is often cis normative, even though there's not really a need for it to be. There's a world of difference between a fluid or non-binary gender identity, and deliberately concealing your gender identity in circumstances where it would be inconvenient.
