For breakfast and lunch, you have the choice of buffet at Cabanas (up on the pool deck - I think it has the same name on all ships) or table service at one of the main dining rooms. There is no set time you have to go as long as you go within the hours they are serving the meal. The main dining room times are shorter than the times that Cabanas is serving.
Additionally, you can do room service 24-7 or you can eat from one of the QS places on the pool deck (again, within stated hours).