While outside during the day, health, magicka, and stamina are reduced, and the regeneration for each of these stats is reduced by 100%. The reduction increases with each stage of vampirism.
http://www.uesp.net/wiki/Skyrim:Vampirism#Weakness_to_Sunlight
What time has sunlight?
Best Answer
http://www.uesp.net/wiki/Skyrim:Vampirism#Weakness_to_Sunlight
That makes 14 hours of sunlight; 10 hours of night-time.