It’s been 2 1/2 years since I got my KS10’s. If I remember correctly, it was about 2 weeks.
Costco has been very good about fixing problems. A couple months ago I was having trouble charging. My local (Prescott, AZ) store updated the firmware. Since then I had trouble only once (possibly from not getting good contact) but otherwise, the charging has worked perfectly since Sep. 21. (Described at Costco apparently fixed my KS10 charge problem).
— Fred