It depends on how much you use them. I was on the Sun last month in an AD and tipped as services were provided. There is a long thread on this on another board and the general rule was around $100.
By the end of the cruise I believe I was very close to that number and we did not use them alot. Had a party in the suite that they set it up, one dinner, and a few breakfasts. They are not part of the general tip pool. There are no published guidelines by NCL.
I have been in the AB Suite on the Dawn twice. Within a couple of hours of boarding I have called the Concierge and let her/him know that I would like to meet and could she/he bring the butlers and room stewards to meet us also. They have come to introduce themselves on that first day. I then give them each some $ and let them know that we will be using their services a considerable amount during the cruise and I would like to thank them in advance. I then let them know that I will also be giving a thank-you tip at the end of the cruise. This was I know that I won't need to keep track each time the Butler comes or the Concierge arranges things. I have gotten great service which I have rewarded at the end. I write a thank-you note and put the money in it.
What I did find out is that I can't tip the room stewards on the first day. First, because they are very busy, and secondly, because I found out that on that first day the cabin duties are shared with other stewards, etc. If you want until the second day you can actually meet those room stewards assigned to your stateroom.
The Concierge is also not included in the tip pool.