[d--b]

Well, once every 10 breaths on average.

But on the other 9 breaths, you get to breath quite a lot of his other farts... So... your breath is really never Caesar-fart-free.

But as a consolation, most of humanity will breath your farts on every breath, so...

[kragen]

Farts are low in nitrogen, but certainly do contain some. And the carbon dioxide, methane, water vapor, etc., are made out of atoms some of which were in Caesar's farts.