I am not immediately aware of any specific rules governing this, so I'll just include what we do at our table...
Firstly, when giving a spirit a physical task the scope of it is important to consider. For example, are you having an earth spirit dig a shallow grave for a dwarf or are you instructing it to dig a mine shaft into a mountain?
For the first scenario, with the spirit digging a shallow grave, we look at the capabilities of the spirit to determine how long that might take. So for a force 6 earth spirit to dig a shallow grave we might allow that within a minute or two (maybe less depending on the circumstances).
For the later, we would rule that as one of those extended services (forgive my terminology, as I don't have my books with me atm) that the spirit could be instructed to take, but forgoes any other services. In this instances it may take weeks or months depending on the specific instructions provided to the spirit.
For a more firm example, the books indicate that using a spirit to assist with reagent gathering uses a service and that can take up to a base time of 10 days. You could also command the same spirit to kill a nest of ghouls huddled in a basement in the barrens... that might only take a minute or two.
It may be a bit abstract, but I think that is actually a strength for the players and the game masters. It allows things to make a logical sort of sense with regard to tasks while still acknowledging the capabilities of the spirits at use.
My two cents.