When are the daily uses of a magic item recharged?

Question

Many magic items in Pathfinder give the user some benefit a number of times per day or for a number of rounds per day. To pick a random example, the Boots of Teleportation state

Any character wearing this footwear may teleport three times per day, exactly as if he had cast the spell of the same name.

Is it ever stated just when used charges are regained? For example, I could imagine it happening at midnight, or after an 8-hour rest.

Two closely related questions:

1. If a magic item changes hands, are the charges used by the previous owner still gone? Or could five different people each use the boots to teleport three times per day?

2. If you have multiple copies of the same item, can you swap them out to get more uses per day?

Answer 1

The rules do not explicitly state a which point during the day an item regains its uses, only that they regain these uses at some point during this time.

So, they could recharge at midnight, noon, sunrise or sunset, or at any other time during the day. It may even be 24 hours after their first use (assuming the day is 24 hours long). Ultimately this is depended on the DM that is running the adventure/campaign. From experience, you will generally find that the approach will be for your items to regain their uses once you have finished your 8-hour rest (in a similar fashion to how spell casters regain their spell slots).

That being said, it is still worth clarifying with you DM for those rare cases when your party does not have the opportunity to take their 8-hour rest (because the world will end in 6 hours) or their rest gets interrupted partway through (and you desperately need to use those boots).

Regarding your additional questions:

1. No. The Boots of Teleportation can only be used a total of three times per day. The Boots do no care who is actually wearing them when a use is expended.
2. Yes. Each pair of Boots of Teleportation has a total of three uses per day. If you have two pairs of boots you can simply put on the second pair when you have exhausted the first and resume teleporting.