This is just idle thought...
I don't think it'd be game-breaking, since you could get as much work done by hiring a laborer to do it or having a slave do it, but...
It should be possible to enchant a wheel or gear to slowly turn itself, with Rego magic. In fact, it looks like it'd be quite low level.
What are some uses for this? I imagine some verditii using this a lot.
Oh, I"m sure that there are a lot of uses for it: travel, grinding/milling, machine parts. A big problem would be mundane reaction to such a device, I'm sure, and the thought that if you are going to spend the time enchanting, you might as well go for the final effect you really want, rather than enchanting parts that will work together to accomplish your goals. Time does seem like it would play a role in this.
If you imagine a water wheel, the base level for moving an amount of wood is 3. If we up that to four for size, then enchant a constant effect into a wheel, we end up with a water wheel that turns even when the stream runs dry for a device level of 4 + 2 magnitudes (Sun duration) + 4 levels (1 for two uses per day and 3 for environmental trigger as per p99 of the core book) = 14.
So yes, these kind of effects are pretty low level (imagine a trip hammer in a blacksmith's forge) but possibly fairly limited.
I love it, what a brilliant idea. This sort of thing is potentially very useful for a small outlay of time and Vis, particular if you have a larger covenant with crops that need milling. Or a smithy that needs regular bellows (although a separate Ignem enchantment would work well). Indeed, it would be interesting to see a range of spells / rituals specifically designed for this sort of use, because it's in saving time doing such things that magic has the most power and interest for Mundanes.... after all, if your peasants aren't farming, they can be fighting a lot more 
"Limitted", in the sense that... you can't split the atom with it, yeah.
Besides anything circular (mills, sharpening stones, grinders, drills and lathes, various other wheels or axles like fans or spindles and such), being able to turn circular motion into piston/reciprocal means that anything that requires hammering/pushing/pulling can be accomplished. Hammering metals, threshing grain, pounding felt, agitating dyes or other baths, rip saws, pumps... limitted only by imagination, as at the advent of the steam engine (which is essentially what you're talking about here, not as bulky as a water wheel and far more portable.) Not to mention being able to gear the motion down to achieve high power for lifting and such, or gear up for high-speed cutting, polishing or grinding, etc.
And if you really, really wanted to go out of paradigm, you could power a cart with it- but I wouldn't go there, myself.
Exactly. As we know, if you can apply power/energy in order to move something, especially in a cyclical motion, your bounded by imagination.
For me, I'd not be keen on doing anything too outside "the paradigm", so no self-powered carts for me.
But doesn't that sound a little unfair? Given that we're talking about magi? Men of vision?
No. If you look at Heron of Alexandria (the real one) he did invent "the steam engine" in some fashion and such was his expertise that he was able to conceive of and build marvels. But that final leap of imagination required to use the energy of the captured steam to make a lasting change on his civilisation's entire technology eluded him.
I'd look to apply the same logic to magi, player or otherwise, unless the troupe really wanted to take a saga away from the medieval feel.
Items created and used in my Sagas have included :
A scythe that'd cut and separate corn and stalk into two different barrels as long as the wielder concentrated at the task at hand. Once concentration was lost the Schythe would regain the ability at next dawn.(Rego Herbam at lv 15) One of the players always mused about doing something similar for Animals... Someone call PETA?
And the spell :
Thaumaturges invisible plowshare...
ReTe (once again can't remember what spell-level that one was since the original campaign was run back in 3rd edition. 
It has been a while in other words. Any suggestions?)
We've also seen spells to temporarily boost fertility in grogs, other spells to ease childbirth and the like. All those were created in a campaign set against the background of colonizing "barbarian lands in the north" where summer is short and the workforce is rare.
... they actually built a building around a Ignem Magus that went into a impressive ignem twillight for a couple of years. "Warms the laboratory bettyer than firewood I say"
As long as there was no smell of burning flesh, I'd agree.
So this is where the Devices which the Dwarfs use originated
(as mentioned in the Terry Pratchett book Thud!)
I'm off to come up with some nifty ideas for my covenant based on moving wheels and gears.
Ohh - this sounds fun - a cart with wheels that never stops from turning.... could make for a nice story....
Sidethought: for travel, you'd probably need to find a way to get it to stop and start. And for general use, you'd need someone to watch the wheel, since neverending turning probably will wear down the surrounding materials quite fast.
As to wearing down the materials of the wheel ,
simply have another enchanted device that constantly generates the materials it is composed of.
Stopping is easy , no traction , no motion.
Levitate the vehicle slightly off the ground.
I think one of the story seeds in City&Guild actually involves just such a magical wheel ... a self-turning millstone IIRC.
As to wearing ... do what they would do and lubricate it. Or make it out of unnantural materials (use a spell, secondary item effect, etc.) so it doesn't wear in normal use. Or just make it really tough like thick bronze or iron.
For that matter enchant it for the 70 year effect and then you have lab notes to make an item legally salebale to mundanes (it runs out - and it is tough enough to take 70 years of operation by virtue of it being superior crafted quality).
For usefulness, I think the best would be a lesser enchanted device that imparts this effect on any wheel (might restrict to any wooden wheel), say, 12 times a day, by touch, Sun duration. Enough to keep a fairly industrious covenant, village or castle going, and you needn't worry overmuch about wear and tear, plus you can switch the usage to anything you like (even the wagon!).
The self turning wheel is fine for the herbam magus, and it is a simple choice for magi who have a strong rego.
I like the idea of a rego aquam enchanment to make a still pond run a water wheel. for the aquam magus. Or perhaps a creo aquam device that creates a river (a river with a very short duration)
I like the idea of using magical bulls and horses to power machinery for the animal magus
Good image- a circular, artificial water run, like an open-top stone aquaduct, or just a moat, that flows perpetually in a circle, with various water wheels located around it, for different purposes (or diff magi?).
Or, like some Escher drawing, that flows up stone/wooden flumes and then drops down over the various water wheels. Hard to explain to mundanes, but very economical on space.
Many people don't realize that a very, very slow trickle of water can supply enough for many folk if it runs 24/7. 1 quart (~1 liter) per minute yields 360 gallons, (~1.5 metric tons) of water/day (or bigger than one of those 2 wheel military water trailers.)