It’s 20% of your rank per week.
You need rank points to rank up. This is based and calculated on the honor per faction on your server. This gives you what is called honor standing. Some servers will have honor caps higher than others due to the amount of people PvPing.
Take for example Nostalrius; most honor caps during the first honor phase were anywhere from 500-600k per week. Eventually you saw honor caps as high as 1 million honor. I doubt very much it will reach these lofty numbers, as the average playerbase will not PvP nearly as much as was required on most private servers.
So the way it is broken down is like this;
R1 - 15 honorable kills
R2 - 2,000 RP (this is determined as stated, on total honor points accumulated).
R3 - 5,000 RP
R4 - 10,000 RP
R5 - 15,000 RP
R6 - 20,000 RP
R7 - 25,000 RP
R8 - 30,000 RP
R9 - 35,000 RP
R10 - 40,000 RP
R11 - 45,000 RP
R12 - 50,000 RP
R13 - 55,000 RP
R14 - 60,000 RP
As you can see, you will lose 20% of something like say 50,000 RP when you are R12 the following week. That is automatically 10,000 RP. That is the equivalent of the difference between R12 and R14. You automatically lose that much RP each week. This is what makes it so brutal to farm. When you are R13, it is more, because your RP is higher. And the same goes for any rank you attain.
This is determined also by what is called a bracket. There are 14 individual brackets, each with separate RP floors. Rank 2 for example has a floor of 400 RP. Probably not much honor to attain, and as such, you won’t decay much week to week. But when you hit say bracket 12, your RP floor is 10,000 RP. This also determines how many people can be in each bracket.
This is why you see such a disparity in the number of R14s, because it is based on a total value of .003%. What that means in terms of server population, is that if you had 10,000 players, only 30 can attain get Rank 14 on a week by week basis. Obviously server sizes will be smaller, so that number is smaller, and there likely isn’t that many people who can stomach that grind each week. Which is why you may only see 2 or 3 new R14s each week per faction.
This is why you see premades of all R12+ players, because they are effectively stacking the high end brackets. They coordinate how much honor they all get, so that they all can achieve the maximum RP per week, which is 13,000. You absolutely cannot get any higher than 13,000 RP every week. This level of communication can eliminate the chance of spending longer grinding RP to rank up, and is why you might hear the term of bracket busting (such as when a player achieves higher RP than a stacked bracket group). These players invariably are either forced into the bracket premades because they have rhe time and effort, or are targeted by cross-faction collusion to eliminate their RP cap potential. This is pretty rare, and also a big tangent from what you have been asking.
Taking into account the total RP cap of 13,000 per week, and the 20% automatic RP decay, the absolute and I mean pure absolute someone can rank to 14 is 12 weeks. 3 months. You can see why some people might spend 6 months to 1 year hitting R14. Work happens, real life happens, PCs blow up, people start stressing, etc. It is an absolutely rough ladder to climb. But that is why the rewards are so good and last until the late stages of PvE progeession for many classes.
I’d like to finish by stating that should you shoot for R14, be prepared.
If you were asking about diminishing returns on per-target honor acquired and not decay, I would also tell you that yes, honor has diminishing returns after a certain number of kills on a target per 24 hours. You cannot farm your buddy’s toon to hit R3 even if you both spent 12 hours a day doing it. After maybe 5 or 10 kills you would receive no honor.
Hope this helps some of you.