I set my "full-time" date sometime early last year, I don't remember exactly when. I said to myself and my therapist "by the end of 2016, if I can hold it together that long." For me, since I'm a worrier and was terrified that it would all go wrong, I wanted enough time to get everything ready -- to get used to my femme name, to come out to people outside work, to work on my presentation, and to get my IDs in order. I also had this idea that, being on HRT, I would experience "male fail" by then, but that has not happened, unfortunately. (E.g., I have breasts, but they're 50AA at best, maybe 50 AAA.)
During that time, I found that my company had guidelines for handling transitioning employees, so I went the "go through HR" route, and contacted them in September. Based on my attorney's experience, I submitted my name change request in August. I started looking for work-appropriate femme clothing in May. So by December, my non-work transition was basically done and the work-related things were scheduled.
So when I showed up at the office on Jan. 4 of this year, it was a total non-issue. People mostly get the name and pronouns right, nobody's asked me any inappropriate questions (or approriate ones, either), and it's like I'd been coming to work this way for years.
I don't know if this would work for everyone, though. Workplaces differ, transitioners differ. Not to mention laws (my office is in New York City, which has lots of legal protections.)