Considering there are currently 38 monster sets, and they're in the store for a month before being switched out, that leaves a lot of time between releases. Especially when we have things like months getting skipped like half of last December and most of February.
They started dropping mid 2018 for a month at a time. As of now, there are about 18 or so that have never been released (though some have been datamined). Only one has returned for a second round, but no other pack was out at the time (Iceheart). But for anyone who's missed a set they wanted, it seems like they're gone for good - that's at least 18 months before they're all through, by which time we'll have more dungeons.
Can the Monster arms sets start doing like the Crates and pop back for a week or so every so often? We've gotten in the habit of getting a week or two's worth of an old Crate season every two months or so (even with the current season still active), and I'd love to see the same happen with the Monster arms sets.