Springer:

On average, patients get about 11 seconds to explain the reasons for their visit before they are interrupted by their doctors. Also, only one in three doctors provides their patients with adequate opportunity to describe their situation.

And

In just over one third of the time (36 per cent), patients were able to put their agendas first. But patients who did get the chance to list their ailments were still interrupted seven out of every ten times, on average within 11 seconds of them starting to speak. In this study, patients who were not interrupted completed their opening statements within about six seconds.