If you stick to the main quest than it's about 40 to 50 hours but if you explore and do side quests than you're looking at 100+ hours and that's just your first playthrough, don't forget each race and class will have different reactions from NPC's and possibly slightly different storylines and even gameplay that will add more replay value, i can roughly estimate that i will be playing around 800 hours just to play the game differently with each race/class and getting different endings that will best fit my characters, this also doesn't include me just wondering around enjoying the world or trying to get all the achievements.
My first playthrough will be as a male Qunari warrior and i plan on doing as much as possible so I'm looking at 100+ hours right from the start, than my second go will be to try out different paths as my Qunari Warrior if I'm not pleased with his ending or decisions, after that i have to play as a female Qunari mage just to see how they look and how people will react to them, plus I'll have to pick the best ending and story i want for her, after that I'm not so sure but i will probably play with each class for each race, so we'll probably be playing hundreds of hours without DLC, nothing to worry about when it comes to length.