In theory a minute or two should be enough to take someones portrait. In real life though, it usually takes a bit longer. Many people can feel a bit uncomfortable in front of the camera. My way of helping them is to be calm, patient and give clear directions. I talk to them and wait for the knots to get untied.
Once we connect and they start trusting me they relax and become less aware of themselves. To me it’s a signal that means we can start working. Now the person I have been waiting for is in front of me, ready to have her picture taken.