In Throne of Bhaal, I made a new Thief. I used the Sandthief's Ring (Ring of Invisibility) initially equipped on my character. This ring has 10 total charges, but its description says it works once per day.
I used all its charges, rested, and got 1 charge back. Maybe I get 10 total charges, then I get 1 extra charge per day thereafter.
I also noticed another Sandthief Ring in my Bag o' Holding. This backup Sandthief Ring still has only 1 daily charge.