There is a huge difference between an employee who sits at the entrance of a lounge which has free muffins in the AM and someone who actually does concierge work, e.g locating theater tickets, good table at restaurant and the like.

The latter is a professional. No tip for simple routine questions and answers. But, when the concierge gets you that table by the window for your anniversary at the restaurant which is booked into 2018, something really substantial is appropriate. If you're spending $1K on dinner, spending $100 on the person who got you in is appropriate.

Lastly, there are places where one may regularly stay and where the concierge knows you and routinely looks out for you. I tend to provide something substantial around the holidays and mid-year.