Okay, so by default, after 15 minutes of inactivity, users disappear from the Members Online Now section and Current Visitors page. However, their last location still shows for a while before disappearing.
For example, "Amaury was last seen: Viewing forum list, 59 minutes ago." After another minute, it'll change from X minutes ago to Today at X (or Yesterday at X if it's close to the next day already), which stays there for a while and then disappears.
Usually I notice that their last location disappears anywhere after an hour and 30 minutes or two hours have passed. However, sometimes I've noticed it disappear only a little after an hour has passed. So what exactly does this depend on?