As /u/pictureandsound said, it's up to you, you direct your transition. But, there are practical concerns, e.g. the effects of HRT, that might force you to come out sooner than you think you're comfortable. There's absolutely no way I could have hidden the physical changes of HRT for more than 4 to 6 months, but I was already out to close friends and some family when I started HRT, so I was ready to go full time before the physical changes forced me to.
I started incorporating women's clothes into my wardrobe after I began therapy and came out to my SO, and after 6 months the only men's clothes I owned were shirts for work, everything else was women's clothes. People both at work and in my personal life thought I was going for a more "metro" look, and while they thought it was a bit weird they didn't suspect one bit that I was transitioning. It wasn't until after I started HRT that people began to notice that I was looking physically more feminine, but most people still didn't make the connection. This, of course, all happened before Bruce Jenner started their very public transition, and I think that if I were just starting to transition now I would approach it a little differently because of that.
*<rant> BJ's transition has really brought a lot of attention to the subtle changes and steps that many of us take early in transition as part of that self-discovery process, and while I do not feel that BJ should be denied that opportunity, I would be lying if I said I wasn't a bit ticked off at them for potentially outing a lot of people in early transition by drawing attention to those subtle in-between steps. </rant> *
Shopping for women's clothes as a male, and incorporating women's clothes into my male presentation really helped build my confidence, and gave me the opportunity to study how people react to subtle differences in appearance. e.g. all it took sometimes was adding a purse and a little mascara and powder to my outfit for people to gender me female, when without those things they would default to male.
I came out to my friends and family after 2mos on HRT, and then went full-time a month later. People at work were shocked; it took them totally by surprise, but my company and my coworkers have by and large been very supportive and accommodating of my transition. It took a while, but I did end up gaining many more friends than I had lost, and I feel like my connections with the people around me are much more authentic, real, genuine.
Over the course of a year I did lose nearly my entire family - some people have remained in contact, but I've spent all the holidays since 4th of July '14 without family so far. I speak to my parents only because I rent from them, but I do not have a relationship with them outside of that. My SO and I split up, but we're still friends and we're remaining together as roommates in order to provide a stable loving home for the kids.... it's hard, but not impossible. Communication is much more important now that it was when we were married.
I am hostage to a relationship that I don't want because my ex and I have kids together - if it were not for the kids I would have given her 60% of my 401k and kicked her and half my debt out of my life months ago - that woman is just barely suffer-able, but I'm making it work because it's just easier than the alternative at the moment. Divorce is hard, expensive, emotional, and heavily weighted in favor of the cis-woman; this goes doubly so when the divorce is due to a gender transition - even in a blue state like mine with civil rights extended to transgender people there are many transphobic people working in the multi-layerd web of state govt. bureaucracy.
I have to get back to work so I'm going to stop rambling here, hopefully this accounting of my experiences help you in your transition, and of course, you're always welcome to PM me as is anyone on the trans subs. As hard as this has been on me, I am still happy that I chose to transition - you can't escape the difficulties of life, they're inevitable, but you can choose whether or not to continue suffering - Life is hard, but I love myself, and that has had profound impacts on my life and relationships that outweigh the hardships by tonnes.