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"Paul Sage: I think, for us, it’s really a matter of making sure the game feels fresh right away. I can’t say more to how it feels after you’ve played 500 hours. No one at the office has played 500 hours."
That's 20 days. They have 250 employees.
The Elder Scrolls Online has been in development for 5 years prior to its announcement in May 2012.
Honestly would not be surprised if there were people working at Zenimax who had never even played an Elder Scrolls game.