When visiting areas with free-roaming wildlife such as American Bison and Texas Longhorns, there are numerous signs warning about keeping a safe distance. They don't, however, actually specify the safe distance.
While I would normally steer clear (pun intended) of predatory animals such as wolves, bears, etc, I would love to photograph some of these large herbivores. However, they can be very protective when threatened:
Though they can appear peaceful and unconcerned, their temperament is unpredictable, and can respond quickly when disturbed. ... Females are very protective of their young, and can become aggressive if threatened.
Is there a general rule or recommended safe distance for non-predatory animals, such that they don't feel threatened?