How many hours of gameplay are there in these games?

Well, technically infinite. Think about how long you can play Morrowind or Oblivion. Now imagine that there are an infinite number of quests to do, imagine there are thousands of dungeons. Really, the game cannot end, because there will always be something to do. As far as the main storyline is concerned, I'd say it's about on par with the length for Morrowind or Oblivion. But, really, I think the real meat of the game lies outside the main story.

I've been playing this game since it came out (15 years now), and I'm not bored with it. I think that says something.

[edit] Oh, right. "this game" = Daggerfall. Were you asking about the entire series?

Depends on your playstyle I'd say.

If you're a power gamer, then all games combined, perhaps 1000 hours or more?

If you're a roleplayer and take your time with different characters in each world - then I imagine you could play all games combined for 10 years and beyond.