There are lots of examples in the Ars Magica book about arcane connections to specific people: locks of hair, bones, talismans etc. There are far fewer examples of connections to objects. We know that Air from a specific place is an arcane connection lasting hours.
What about soil (earth) from a specific place?
What about a stone chip from a specific, natural boulder?
What about water from a body of water, such as a river or lake?
What about a piece of a man-made object (a wooden chip from a ship, a clay chip from a pot)?
What about a "symbolically central" piece of a man-made object (a nail from a coffin, the cornerstone of a building)?
Well, rock or metal from a specific place is Years, so if the soil is rocky (contains pebbles) you can go with it. Otherwise I would go with Month, since loose earth is just not as fixed.
I would go with the hair/feather/scale analogy: if you just take a small chip, one that won't be noticed without close examination, Months. If you take a bigger chip, one that would be visible from a distance, Years. If you take a significant portion of the total mass (say, 10%), Decades
Water from a moving body of water is Hours. Water from a still body of water is Days. That's in the guidelines
I would say Months, as per hair/feather from a Bird/ scale from a Reptile. Years if a significant portion was taken (lock of hair / group of feather scales). Possibly decades for a really significant (in a functional way) piece of the object: the keel or mast of a ship, the clapper of a bell, the cornerstone of a building, the first nail from a coffin (the others are not really significant). (Body Part)
I would equate them with wood shard, for Months. Except a fallen leaf or twig is only worth 'Hours', as in shed skin.