Depends on the circumstance John. A bull in Musth will leak pretty much continously and is usually a lot more aggressive than a bull that is not in musth. (There's also a lot of other symptoms related to Musth)

However, the gland also leaks at other times on both bulls and cows. Most of the experts seem to think the leaks also occur with mood changes or when they feel threatened or pressured etc.

From my experience, it also seems to happen a lot more in adults than in youngsters or adolescents.

I It's a while since I've read the book but seem to remember it's discussed in "When Elephants Weep" by Jeffrey Masson & Susan McCarthy. Also in the Douglas Hamilton books and probably also in "The Sound of Thunder" by Katy Payne. - All good books and well worth reading if someone wants to try to understand a little about Elephants before they hunt them....