I've played for 60 hours now and have yet to complete a single quest outside of Whiterun Hold.
This.
If you:
- Walk everywhere instead of fast-travelling
- Mix in random side-quests, activities, and exploration with main questline progress
...the game will last for hundreds of hours.
If you:
- Use fast-travel to beam yourself everywhere
- Rush through the main questline and other major questlines, leaving only minor sidequests
- Take advantage of cheap ways to become powerful quickly (like smithing in it's present state)
...you'll likely start losing interest in the game within the first 50 hours.