All the time, if there's so one around. And I do not merely talk to myself.
I frequently argue, loudly, with myself.
I explain things to myself, but I never listen to reason.
I mutter in agreement with myself.
I laugh at my own jokes.
Sometimes I slap myself on the back.
Sometimes I slap myself upside the head.
And sometimes I just verbalize the weird whatnots currently running through my head.