I've seen this question come up recently as well as witnessing some confusion related to it, mostly around how MuVi works. Just how long can a Momentary spell last? As a basic answer, long enough to complete its effect, but not very long. That's pretty vague, though. In some cases, it certainly may be a very small fraction of a round, noticeably less than a second. But what about the upper end? While people did discuss this quite a few years ago in a thread, that was also prior to some changes to MuVi, so I hope to add some clarity without doing threadomancy.
First we should note that a round is approximately 6 s. Though seemingly always buried, this is stated many times across a number of books.
Now, let's look at some example spells. In the core book we have:
Dust to Dust - It takes two rounds to finish its effect. I've heard comments about the spell lasting one round and then natural decay happens for one round. Anyone who has any experience with things decaying knows that even with only 1% remaining to decay, that 1% will still not decay to dust in only 6 seconds. Even with far less than this, say one eyelash remaining, it will still not decay to dust in only 6 seconds naturally. So we know for sure this isn't natural decay, and the spell lasts about two rounds.
Gift of the Frog's Legs - The leap must be made within 10 seconds, so a little shy of two rounds.
Curse of the Rotted Wood - With smaller things this can take just a fraction of a round. But with the biggest sizes it affects, it takes up to two rounds for the same effect. So we know this can last up to two rounds.
Heat of the Searing Forge - It does damage across three rounds. I've heard comments about that being natural cooling. Anyone who has worked with hot metal knows that the metal will not cool from too hot to handle with that scale of damage to perfectly safe to handle without any harm in 12 seconds. So we know for sure this really isn't natural cooling. Now, how much of the first round remains is still in question, so this lasts a little over two rounds.
Crest of the Earth Wave - It travels at 50 paces/round and can reach 50 paces with a shout, so a full round.
All of MuVi - The default (not fast-casting) to use MuVi on your own spells is to cast it one round and the spell to be affected the next round. The MuVi spell's duration must fully encompass the casting of the second spell, which theoretically could end a little over a round after the MuVi spell has been cast. So just over a round.
So we can see some consistency. A little over a round is OK, so long as it's not much more and the magic is just finishing its relatively immediate effect (the next jump, the next spell, finish decaying, finish cooling down, etc.).