Looks like this time around DE swapped the global health pool for a preset decaying one. You've got a bit under three days for each one by my calculation, and a new one shows up at 3:15 PM EST each day. Torment isn't even below 80% after 12 hours up, he normally is dead before that, so it's gotta be a fixed rate, especially since they drop at the same rate whether hidden or not.
Also some speculation that their node visibility is on a timer, since it seems to come up in roughly the same intervals. That, and it seems like they're showing up a whole lot on a limited set of nodes. Mostly Akkad, Hydron, Spy nodes, and low-level exterminations, so I kinda suspect even the nodes aren't random, as that looks a lot like a mix of easy-but-not-rushable and "here newbies, learn the basics of some of the usual starchart stumbling blocks if you want loot".
Basically there's not much rush this time around, and you can run 3-4 missions for each period of visibility since they don't get instantly gibbed whenever they appear. It's ~4% drop rate for Argon Scope and Memeing Strike, but without true global healthpools that's actually a realistic goal. Even if you miss 'em, the market is crashing hard now that people know about the change to the event.