I've been thinking of this more lately, especially based on my experience.
I submitted my name change application 5 weeks ago, though I had been using the new name for longer than that (I had to wait for a replacement birth certificate). And as of April 1, I only go by my new name. However, I was still using men's washrooms and change room facilities. However, over the weeks/months, I became increasingly very uncomfortable doing that, even though I wasn't fully suppressing my male secondary sex characteristics (e.g., facial hair). But around May 1, I had a conversation with a colleague at work (my department chair: I'm a professor), and just realized that I'm already "presenting female" full time, even if I'm not suppressing all of the secondary male characteristics. My manner of dress, social interactions, pronouns, name, and everything were consistent with a female gender, and so it made sense to stop using men's facilities even if I weren't suppressing the facial hair etc.. Shortly thereafter I had my gender designation changed on my driver's license (I mean by a matter of days).
That's the day I consider starting "full time" living. Everyone knows me by my female identity now and refers to me that way; I only use women's washrooms and change rooms (though it took another week before my sports club made their policy decision and announcement, with my guidance, I'm happy to say). So I consider myself full time even if I don't hide the beard shadow everyday, or if I don't wear my breast forms everyday.
It all raises an interesting question for what constitutes "beginning" RLE.