Another n=1: no timer, I wouldn't set your own because it would make interacting with the interviewer a bit awkward (good chance to stand out in a bad way from all the other applicants)
They will tell you when it is time to switch, at that point just thank your interviewer and leave, they don't hold it against you if you haven't finished your thought yet as long as you've said something coherent in the time you've had (this is an MMI, not a rap battle)