So I'm currently in the midst of running around doing side quests for the same amount of time as it took to beat the main story until the final day happens. (the-games-blog.com)
What makes this even worse is that doing side quests changes parts of the game and can influence endings, so if you skip them, you might not get as good an ending. (darkstation.com)
All in all you can play the main story in 10 hours or wander off and put in 100 hours doing side quests so the choice is yours. (metacritic.com)