>>528572628
23 missions. I started from launch and finished just now, so took like 11 hours? Minus an hour break. I also did an extra heresy mission by mistake, so you could just do it in 22 missions, I believe.

For the first ten or so I did the "special event" until I got bored of it and then just did quick play. Maxed the new shotgun and the maul+shield, then started fucking about with the power maul.