According to the American Hotel and Lodging Association, you should tip the same for room service as if you were dining in a hotel restaurant. They recommend a 15 to 20% tip.
The AHLA Gratuity Guide was published in 2014, and Americans have increased tip percentages since then to 25%. This means room service tips should probably be between 20 to 25%.
Hotels sometimes add a gratuity on room service checks, and in this case, there's no need to tip extra. If that gratuity is low, however, you may want to add a little more.
These tipping rules apply mostly to North American countries. Tipping etiquette may differ if you're ordering room service at a hotel in another country.
Many European countries tip less, or not at all, and in Japan and China, tipping is less common. If you're traveling, it's best to research tipping customs.