My first character has barely completed anything after 200 hours. That's how long it took to complete 50-60% of thieves' guild, 2 daedric quests and learn the final word of Unrelenting Force from the Greybeards.
I switched to a warrior just to mix it up and try a different playing style. So far this character has taken about 80 hours or so to complete
Spoiler most of the Companions quests and help the Stormcloaks capture Whiterun. Only did the main quest up to the part where you learn Unrelenting Force. No daedric quests so far.
If I were to focus on the main quest, civil war, one or two other faction questlines and a few daedric quests, this seems like it will take at least 400-500 hours. I have a friend who has somehow managed to play Skyrim about 700 hours since release date, 600 of which was his first character, and he did not even touch the civil war or main quest.
I believe playing time should be shorter on an easier difficulty setting (playing on master) and if you use fast travel frequently. I fast travel primarily to drop off loot at home after clearing dungeons or to teleport to the interior of a town when I am already close by. Other than that, I typically ride a horse or walk.