Ok, so as far as I see, I skipped about 8K rep because I did the Teldrassil quests too late. Fine. If I add that to the current rep I've got (revered 5K), I would still need 8K rep to be exalted.
So I need to farm morrowgrain to gather 8K rep.
I've made a test, and collected 100 un'goro soils. I also created a herbalist alt and done all consuming with that for better droprate. After consuming seeds in every 10 minutes for almost 10 hours, I got 53 morrowgrain. That is about 1 morrowgrain / 10 minutes ratio.
100 soils needed 1.5 hours to farm (usually just running around in un'goro and looting dirt piles - way faster than killing mobs - dirt piles respaws pretty fast), and did the consuming at work (kept pushing a button on my laptop in every 10 minutes - I hope that doesnt count as botting

.
At the end I farmed 5x75 = 375 rep with darnassus in 11.5 hours!
If I played on my non-herbalist main, the droprate was about 3 morrowgrain from 10 consumes, means almost 5 hour for 75 rep, so either I'd need to create a new acc for the alt and doing the consuming parallel (which is considered multiboxing - ie. against terms of use) or log off - log in every 10 minutes.
So consuming morrowgrain while farming soils or questing is a no go because of the extremely low drop-rate for non-herbalists.
But still with the herb alt, it would take 213 hours of game time to gather 8K rep from morrowgrain that needed. That means 8-10 days of constant playing without interruption, so calculating with a more realistic 3 hours of playing / day = 71 days of farming.
But I need 16K rep, so if I'd spend all my time on farming soils and consuming, I'd need 142 days (aprox. 5 months) to get all morrowgrain. If I'd do anything else in the game (ie. playing on my main), the droprate would drastically decrease, so the time would go up to 426 days: more than a year!
So let's be realistic: if you do _anything_ else in the game than darnassus quests from lvl1 or if you skip few quests on the way, or you are not botting, you practically have no chance to be exalted with darnassus before the 1.5 patch.