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)?
For all these, and any more, make the closest analogy to the examples in the book. They were not (completely) chosen at random - they are a good representation of the spectrum of things that can work.
If in doubt, talk to your Troupe, so everyone feels they are on the same page. Their input and opinion is far more important than ours.
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.