i have one dog who i have been smoking with for over 9 years, hes 10 years old. I dont think he ever maxes out on it, i basically give him hits until he walks away.
i have another dog who is about 12 lbs and she gets a little nuts, but i cant say a definite amount of hits- it just kind of varies, what is being smoked, how big the hit etc. so as soon as i see her eyes get a little reddish, i take her out of the rotation.
i will say tho that after a decade of smoking weed my dog has had no detriment, is outrageously healthy, and still cant get enough ganj.