The often quoted study on how quickly doctors interrupt patients comes from 1999 (JAMA. Jan 20, 1999; 283-287).
On average, doctors interrupted patients for the first time (during a medical interview) after only 23 seconds.
Interestingly, the study also showed that most patients only have 2-3 main concerns to discuss and, if given the chance, will state their concerns in an average of 32 seconds.
A more recent study has been done (March 2007).
It showed that now doctors interrupt their patients within 18 seconds of the start of the medical interview.
It's not all bad news for those glass half-full folks. 44% of physicians who were given training in conducting open interviews were found to allow patients to express all their concerns, compared to only 22% of those without the training.
It's not so good news for the glass half-empty folks...56% of physicians who were given the special training still interrupted.