I don't think about it but it does differ depending on the song. I gues now tha tyou ask it tends to be about every bar - 2 beats in , 2 beats out so it would tend to be about like you breathe.
I do get amped up when I play but it is like 'runners high' amped or something. When I play it is all about the music, the Zone and the people for me. I tend to go deep within to break out of the box into that next level. I try to come in like I am preparing for a football game readying for that first hit/collision to determine the REAL approach for the game. I am more comfortable laying back behind the beat but I hope to get better with my fast tempo play which requires a different breathing.
As I get back into athletic shape my breathing will tend to be to the whole note... in 4 beats out 4 beats.