A full quest guide can be found here:

http://www.wowhead.com/guide=42/the-stuff-of-legends-an-in-depth-guide-to-dragonwrath-tarecgosas-rest

The biggest problem with completing this quest is time vs manpower.  There is a large discrepenacy between 25m vs 10m in how long it takes to complete.

First phase - 25 embers

10m 0-1  (7 bosses = 4 weeks  (4 raids) to complete)

25m 1-3  (2 weeks on average to complete)

Second phase - 1000 Seething Cinders

10m  normal: 15-25  heroic 20-30

25m  normal: 45-55   heroic 55-65

completion time:

10m normal - 140 cinders per week, 8 weeks on average to complete

10m heroic - 175 cinders per week, 6 weeks on average to complete

25m normal - 350 cinders per week, 3 weeks to complete

25m heroic - 420 cinders per week, 3 weeks to complete

Third phase - 250 Smoldering essences, obtained from the corpses of bosses in firelands, except Ragnaros.

10m normal: 4-6  heroic: 7-8
25m normal: 7-8  heroic: 9-11

completion times:

10m normal: 30 per week, 9 weeks to complete.  heroic: 42 per week, 6 weeks to complete

25m normal: 42 per week, 6 weeks to complete.  heroic: 60 per week, 5 weeks to complete

Now, in adding these times up, I did not suggest completing the quest and starting the next phase in the middle of the run.  If you were to try to be as efficient as possible, turning in as soon as you were done and starting the next phase mid run, you might shave a weeks off the entire completion time.  I am looking at 16-17 weeks to complete (phase 2 and 3), 11-12 weeks if we could kill all the bosses on heroic 10m.  25m normal is about 8-9 weeks.

Overall, i do not expect to complete this before the next expansion unless Blizzard changes the quest chain.