I would probably judge that on a case by case basis. Usually, you use such an item to get around some problem. A magical key that opens a door, underwater breathing spells to dive to Atlantis, and so forth. The spell will last as long as the obstacle it was meant to overcome lasts. The problem is overcome, and unless it is interesting to make it a perpetual thing, it moves to the background.
If it is something that's going to be interesting if it comes up again, I think it works best as a compel than a fixed duration, because you can time the impact on the story to be in the most critical moment. You need to leave Atlantis in a hurry, and just your luck, you feel the spell slip as you run towards the airlock. This works best as a compel though, I think.
Or if it just wouldn't make sense to have lasted. Like if you stay a few weeks in Atlantis, the spell is gone for sure. In edge cases, see above. Is it going to be interesting to have the spell fail now? Do it.
