Ah, I see. Well, spirit power use is definitely covered by RAW on page 302 of SR5: "If the power is sustained, it counts as one service no matter how long itís sustained." So at least in the "use the Concealment power on me" scenario, that'll last until the spirit is no longer bound as far as RAW is concerned.
The other scenario of "guard this warehouse" could theoretically count as a combat service which has a duration of one fight counting as one service; so if a Force 6 spirit that owes three services is guarding a warehouse it would continue to do so until 3 fights had occurred, or until the summoner bound the spirit again to (hopefully) accumulate more services. What exactly counts as a "fight" would depend on the wording used; a high force spirit that doesn't want to spend all eternity guarding a warehouse could, in my opinion, decide that protecting the warehouse meant taking out those nasty devil rats trying to make entry, and could be considered free of its obligation once (number of services owed) separate fights with said devil rats had occurred.
That being said, I don't think I've ever run into a player trying to abuse this kind of magic to an unreasonable degree, but I guess munchkins will be munchkins.