If you plan to have your breakfast/lunch there most days of your cruise, at the end of the cruise is a good time to tip.
IMO, if you will only eat there once or twice during the cruise, I would tip at the end of each meal.
Just my personal choice.....I am sure they would be appreciative of any tips you extend to them whenever it is you opt to do it.
It is a very beautiful restaurant and the service is wonderful. Enjoy....we sure did and are looking forward to doing the same very soon again.