Originally Posted by
Rixis
12 Titans Runestones, last 6 bosses of ToT and first 8 of SoO. Then Heart of the Thunder King (off lei shen). Takes about 3 week to complete currently, so 3-4 week of runs next expansion if you get pugs that can clear every boss you need. Sha of Pride and Lei Shen are 100% drop for the Runestones so 6 week tops for those. You can get his heart even after already having killed him that week (via LFR), not next expansion though.
Edit: Oh, you also need to kill all 4 celestials too for the last stage of the quest, depending on class, might be a pain in the arse to do, and as no mounts drop from them, little incentive for people to do them next expansion, so you may need to grab a friend or two. Dunno how awkward they'll be next expansion.