Uhm. Hard to say how often I do it. I do it quite often I believe. I'm a introvert person so when I am alone and thinking for myself sometimes I just talk out loud instead of think, and then I realize I did it and feel retarded Most likely I do it when I am thinking of a two person conversation between myself and some random person, a friend for example. I then think for a while and poff, my sentances are spoken out loud randomly instead of thought of.
Probably sounds weird. But I think a lot over situations and consequences of actions. I think many people with quite an intellect does the same thing. So talking out loud is just a consequence of my brain thinking for so long that it believes it is a real conversation.
Basically when I think to myself I do it. And when I imagine talking to other people I do it. I also do it sometimes when I get pissed of at something, most likely a game. But that's more "sane", so that's not what I think you mean when you mean to "talk to yourself".