Aside from being in the middle of combat you can turn off the game from anywhere and you'll load back exactly where you left off. Bonfires aren't checkpoints, you don't have to be anywhere near one to save your progress and leave the game. Your point about losing momentum is dependent on the person playing the game, not an issue with the game itself. I've never had an issue with loading into a game for 10 minutes at a time. Someone else responded that boss fights are too long, but I cant recall one that took more than 5-10 minutes.

Strangely, your experience with Dark Souls is my experience with many rogue lites. You generally cannot leave in the middle of a run without losing all progress for it. You either have to win or die for it to count. And if you're set on winning, then those games can last upwards to that 45 minutes you mentioned. The games I play (Spelunky and Vagante) do not allow you to resume your current game when you shut down and return later.