This is pretty basic stuff, but all the more reason to get it right once and for all.
In what circumstances are the effects of a Rego spell lasting, even after the duration of the spell runs out (and, thus, what Rego effects can be meaningfully done with Mom duration)?
Rego is supposed to change the state of an object into another that it can naturally have. This is pretty straightforward, at least in theory. Muto puts objects into states they cannot naturally have, and thus the effects revert once the magic ceases.
So, is this the rule of thumb to follow: "When a spell's duration ceases, any unnatural state of an object will revert, whereas any natural state will remain?"
This would mean that when a spell ceases,
Objects moved around with Rego fall to the ground where they are
Objects made malleable with Muto cease to be malleable but retain their shape (insofar as that shape is naturally stable)
Something teleported by Rego remains where it is
A rock turned into bird with Muto reverts to the original rock (the shape of the bird having nothing to do with the shape of the rock) and falls to the ground where it is
A statue made mobile by Muto stops moving, retaining whatever pose it was currently in, because its pose is a state the statue can naturally have
A longsword tied into a knot by Rego remains that way, since it could be made that way by a blacksmith
A person put to sleep by Rego Mentem stays asleep if he was tired to begin with, otherwise wakes up in a natural manner
An animal calmed with Rego remains calm until aroused
I think the above makes perfect sense, however look at The Forgiving Earth and The Unyielding Earth (ReTe20, ArM5 p.156). As they stand, they're both duration Sun. What gives? Why couldn't they be Mom? It's not changing the substance (that'd be Muto), it's just reproducing what a plow or a herd of elephants (respectively) might naturally accomplish.
Did I miss something?
To answer your question, by using a longer duration, rego effects will maintain the spells effect even if another external natural effect would alter it.
In other words, the dirt will remain loose or pacted even if rained uppon or tapped by horses. The spells could be reduced to momentary durations & would effectively work well.