Okay... what do I want?
First, what substances are worth distinguishing?
Liquids: easiest. Even a minor mage should be able to make changes in a few rounds.
Flesh/Soft Wood: a bit harder. Making changes without doing damage to a living tree or person is much harder.
Soft Metal: harder still
Hard Metal: hardest.
How about some decent defaults...
Liquids should take rounds.
Wood should take twice as many.
Soft metal takes minutes.
Hard metal takes 10s of minutes, often hours.
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