Tired during the day? How long a nap should I take at lunch?

Just long enough. Everyone is different. Some people never nap. Others require a siesta. If you have excess daytime somnolence - that is you are more tired than the average joe, wake up without feeling refreshed, could fall asleep at the drop of a hat when not stimulated.... Or you have heroic snoring or paused breathing while asleep. You should see your doc to consider sleep apnea.