Hello! I have a question: How longwould it take to complete Main Quest, Side Quests, DLC Quests, Daedric Quests, get every perk possible, clear every dungeon, etc. just complete 100% of the game? Also, I won't use fast travel or carriage, just feet and horses. So, about how much time will it take? I think it will be something around 200-300hours, but what do you guys think?

This way of thinking is totally, completely, opposite to way I think about playing Elder Scrolls games. But I'll take a guess anyway. For you, I'd guess your estimate of 200-300 hours might be about right. For someone else it might take a lot longer.

It depends on a playstyle. Fast-travel, fast-forwarding dialogues or not minding the books will shrink the playtime. I once had a character with which I used Fast-Travel all the time and omitted most books (though I listened to all dialogues carefully). By the 85 hours mark, I already completed all Vanilla questlines and got almost halfway through Dawnguard.