Room service is available 24 hours a day. You can order breakfast, lunch and dinner. They also have snack such as sandwiches, hot dogs, hamburgers and fries. There is a menu in your Guest Book that you will find in your cabin.
Independence of the seas 4/7/2013
Explorer of the seas March 11,2014
If you want something from Room Service that's not on the menu, they will usually bring it. We often wrote our request on our breakfast menu, or just called the room service staff to ask. Always found them very helpful.
If you're traveling with children, you can order snacks or beverages to keep in your refrigerator in case someone wakes up early. (Juice, milk etc.) We also liked it for afternoon snacks such as fruit, crackers and cheese or sandwiches.
We didn't use the room service often, but it's very helpful, and all included. On RCCL Vision it was very prompt. Last year, on Star Princess I found that it took longer to arrive so wasn't as useful. (Faster to just run up to the buffet and get what you want.)