Just like with a hotel, are you going to use his/her services? I tried to get one of them to give me guidelines by talking about another concierge I was cruising with on an upcoming cruise. Still struck out! But if you use the other tipping guidelines as a reference I am sure you will come up with an appropriate daily amount. Multiply it out to present it the last evening of your trip.
The key to the Concierge lounge should be waiting for you in your suite so I suggest going in and meeting the concierge as soon as possible. Introduce yourselves and decide what services you will use. I've never had cocktails from them but it is a major perk. I use the room for the specialty coffees, the morning Continental breakfast and just to socialize (the Radiance class ships have floor-to-ceiling window, unlike Voyager class which is inside). Bottom line? I still tipped $3 per day, just because I appreciated how they oversee the lounge.
P.S. If you have a problem with anything, the Concierge can and will make the call(s) for you.
I tip at the end of the cruise. Depends what services we use. I always keep twenty one dollar bills in the cabin so I can give a couple for room service but everything else gets tipped at the end of the week. Spa service get tipped on the room charge for each service.
We just cruised on Radiance of the Seas. The concierge, Joanne, was wonderful. We only used it in the evenings, usually between 6 and 8pm, for our cocktails before dinner. Did not have to ask for my very dry martini, made perfectly, or my wife's cosmo. She just knew! We also had her make our dinner reservations for Portofino and Chops, where we ate all our dinners, alternating each evening between the two restaurants. She was able to get us the times and table we requested, after we were initially told by Matre' d that our requests were sometimes not available. She was very sweet and attentive to our needs. For this reason, we felt a very generous tip was in order, and we tipped her $80, and tipped her assistant an additional $30. We always reward those who deserve it. They both were very appreciative.