One gamer has rushed through the original Dark Souls in a staggering 49:55 minutes. The time was posted by Kahmul78 and you can view the entire run on Twitch should you be interested in getting some pointers on how to improve your own efficiency (incredibly, Kahmul78 reckons there's plenty of room for improvement - though we'll have to take his word on that).
So, Dark Souls can be completed in less than 50 minutes (without cheats in this case). Now all Kahmul78 needs to do is finish the game in that time using nothing but a Rock Band guitar.