I think I'll go with most of the time because, as usual, it depends on my surroundings and what game I'm playing.
If it's a horror game, then I'll definitely keep a string of nervous sentences flowing just to make sure that I don't get too creeped out... I talk as a response to fear, though most of it comes out more or less as a garbled explanation on how the bad guys should just turn around and go home because I'm not worth bothering.
Of course, as with most other gamers, I'll also release torrents of cursing when something kicks my ass enough times or I feel that the game's being "unfair" to me. If I play a protagonist that doesn't talk or has lines made for them (Like Dragon Age Origins), I'll often voice act for them (purely because that's what I want to do) and if it's a silent hero otherwise, I'll often try to crack jokes.
I find myself "talking" more or less to my video games or characters if there's someone else in the room in an attempt to make someone laugh with my cheesy dialogue or ranting monologues about how I find it amusing that the characters will do something.
I think one reason why I talk so much is because I long to make a "Let's Play", so everything I do until I actually make one is something close to practice.