Background: Agent Jones is pretty close to being peak human: in physical contests he wouldn't win against an Olympic medalist in their discipline but he's easily above the 95 percentile when it comes to fitness. In terms of mental skills, he's sharp and has two decades of exemplary in-the-field secret agent experience. He also lives a very regimented life with exact rituals concerning what, when, and how much he eats, how long he sleeps, when he exercises, when he shaves, etc. Needless to say, he knows himself and his body very well.
The Scenario: Somehow, the evil bad-guy syndicate gets the jump on Jones, and the last thing he remembers before waking up is getting manhandled into a private jet before fading into a drug-induced unconsciousness. Now he's woken up in an interrogation room.
The Question: Being as in-tune with his body as Agent Jones is, can he estimate how many hours or days he was knocked out by the amount that his hair, specifically the stubble of his normally clean-shaven chin, has grown? What level of accuracy can be expected for this estimate?