Is there a set definition for this?
Does it basically mean living like a binary male where one is dressing and behaving with typical male attributes?  I was asked this question by a doctor in a form that I need to fill but I don't know if ONLY binding myself ever since the appearance of breasts and having my gender marker changed to M counts as "full time" and I don't want to lie if this doesn't apply to me since I am obviously (in my appearance) nonbinary. 

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. 

Thank you for the explanation Bran. I was just never asked this question before and I didn't know how else to respond but to be honest like how I define my maleness if they are open minded enough to actually believe what I say and take my word for it.
My name isn't gender specific so I am fine with that but the gender marker is already M. With the actual pronouns though, I am usually assumed as a she by people who see me  I am guessing more than 50% of the time? I can't say for sure since I avoid people whenever I can but I can tell that they are awkwardly confused when they hear me talk.