Depends! Marta usually has to go in a few times because of something, Adele is barely ever sick and only goes in once a year, Vallis has had to go in a few times already and we've only had him 2 months! Seine has been in twice this last year, once for her shots, once to get spayed...

If they are completely healthy, at least once per year, but something usually happens and they need to go more often.