Currently the Jade and Tyrant plate gear is ranked such that it can reasonably fetch 5000 gold for the chest/leg slots, and 1000-2500 gold for the other slots. I can tell you from my own personal experience dealing with these particular sets, that it's very difficult to collect a full set of this gear. I think in the 6+ months I've been collecting transmog pieces, I've come across 2 pieces of the Jade set and 3-4 pieces of the Tyrant set. By contrast, Leatherworkers can make items that look exactly like the Jade & Tyrant chest, gloves and bracers. On top of that, the crafted versions are mail, not plate. So this opens up the look to a new group of players.
A newly minted Death Knight just making their way out from Eastern Plaguelands will start at Neutral reputation with the Cenarion Circle. I used this as my test case to see how difficult it would be to hit Revered with Cenarion on a character with no previous standing. My first stop was Silithus - the most prominent area for the Cenarion Circle. You can complete all 20 available quests there and get to Honored in about 2 hours (less if on a max level toon with a flying mount). The quests will grant about 9450 reputation, plus another 800-1000 rep for killing the local Twilight mobs while doing the quests. They also provide one repeatable quest: Encrypted Twilight Texts. From my test run I was able to clear a twilight camp in about 5 minutes on my level 85, plus collect about 10 encrypted twilight texts that can be turned in for 500 rep. You can also kill mobs in AQ-20 or AQ-40 if you prefer to farm raid content.
If you plan to keep going to Revered you should also visit Desolace. Back in the day Desolace didn't have any Cenarion quests that I recall, but thanks to the new oasis area there are quests available now. There are two main quest hubs of interest, Cenarion Wildlands and Ethel Rethor. Do all of the quests at the Cenarion Wildlands until they send you to Thargad's Camp. At Thargad's Camp, do his two quests, then the follow-up until he sends you back to Karnum's Glade for a few more quests. You want to complete all quests up through Cleansing our Crevasse. You can do the quests at Ethel Rethor at any point, they are not unlocked by other quests.
In total you will end up doing 28 quests at these two hubs, fifteen of which give Cenarion Circle reputation. Wowhead is reporting that these quests are worth 250 reputation each, but I received 500 reputation for all but two of the quests. Assuming you complete all fifteen Cenarion quests, you can get 6800 reputation in Desolace (not counting bonus rep gains). This should take you at least halfway to Revered.
If you still need rep at this point, you can pick up a handful of quests in Eastern and Western Plaguelands. There are two druid NPCs near Mender's Stead in WPL who will give you three quests. Unfortunately you also have to do the quests for the Draenei quest NPC to unlock the last quest they have. You can skip that if you want, but to complete all available Cenarion quests you have to follow the chain up through Return to the Stead.
It's debatable whether the WPL quest hub is worth the 1900 reputation gain considering the extra half-dozen quests you must finish. It may be more efficient to do the first two and then move on. In the Eastern Plaguelands are a couple more quests which you can pick up near the Fungal Vale (NW of the Crown Guard Tower). These two quests can be done pretty quickly and reward 500 rep each.
There are also a handful of Cenarion quests in Un'Goro. Unfortunately I don't think these are worth bothering with unless you are also doing Loremaster. The progression on the new quests in Un'Goro is not well documented on Wowhead. There's a troll NPC at Marshall's Stand who is said to have Cenarion quests available, but after completing all quests at that hub I still hadn't managed to unlock him. Given the fact that I did close to a dozen quests, I don't think this particular location is worth the time investment. You'll do better to spend 20-30 minutes in Silithus farming Twilight mobs and texts.
I see no reason that the typical player couldn't go from Neutral to Revered in 4-5 hours. This assumes you spend about an hour farming mobs and twilight texts once you exhaust all of the worthwhile quest options. This seems like a fairly low time investment given the sort of grinding that used to be required back in the day. I still remember when the bugs in Silithus were elite and you had to do most of the Cenarion grind as a group :)