Welp, ignoring the math, I just ran some tests. With myself and a couple others in my fellowship watching, we estimated the buff was up ~90% of the time. Sometimes it wouldn't quite refresh before running out, and sometimes it would refresh within 3 seconds of the last application (so 17 seconds left turned into 20 seconds left).

It helps that autoattacks count as a Harmful Skill, so you can actually get an attack every second or so.