Consequently, how many hours is Dawnguard?
If you're talking, exploring, experimenting with new features and doing the side quests, then it's more like 15-20 hours, I'd say. 7-8ish hours as expert, while doing a couple side quests.
Also, which is better Dawnguard or Dragonborn? Dawnguard has better story/characters. Dragonborn has better landscapes and features.
Keeping this in view, what level should you be to do Dawnguard?
To start the Dawnguard main quest (and also the side quests) your character must either be at level 10 or above, or you need to go to Fort Dawnguard.
Are the Dawnguard quests infinite?
They are infinite. Gunmar and 2 or 3 other Dawnguard NPCs will give random quests. Once you complete a quest for one of them, the game randomly creates another one for one of the NPCs.