I was able to complete the legendary rogue dagger questline this week with my rogue Shylo. It took a couple of months (a little over 2) and 10,000 gold. It seems the easiest to achieve of all the legendaries, the Shadowmourne questline took 4-5 months as well as a couple of very tricky encounters (especially solo, and one required 5-6 people to help.) Still pretty good though because I was more PVP focused during Cataclysm and missed out on the later raid tiers, so it was cool to see the story of Deathwing and the dragon aspects.
I’ll get to use them the next time there is a Time Walking Dungeon Weekend.