I used to work at a hotel and everyone tips very differently. $5 bucks used to be the going rate for a small task, and $20 for a bigger one. For example, a simple dinner reservation would only require a small thank you. If the Concierge was to get some tickets for you to a show, that would qualify for a $5 tip. If the Concierge had to go out of his or her way to get something done FAST, like pick up flowers for you, then a $20 would be appropriate. Just keep in mind that a lot of times Concierges also make money on the back end, so be thankful, but you shouldn't have to overdo it.