Lately, I've noticed that while reviewing and learning, I yawn like a madman. I'm taking this means I need to take a break, as my recall rate has been slightly lowering. (Used to avarage on ~85%, it is now 70-75, which is too low IMO)
Now, the question is: Can it hurt to take a 2-3 day break? I do intend to at least try to keep the expired cards from piling up, but no more new kanji for a bit. (I mean, I avarage at around 100 reviews a day, which is only half an hour work or so)

I'm mostly concerned that this'll ruin my momentum, and perhaps even my motivation, although the latter is unlikely. Still, it'd be nice to hear from other people who have taken short breaks whether or not it's worth it.

I know exactly where you're coming from: when I passed the 500 kanji mark my retention rate started to suffer. But I am TERRIFIED of taking a break!

Haven't dared yet.

yawning is actually a way of stretching and waking up your mind....keep doing it when you feel like it....try to do it even more... if you can try not to take a break...I did and I have 800 failed kanji now....just set a small goal until you pick up steam again and always try to keep up with your reviews...it will save you more work in the future

Taking a break is discouraged, but it's not prohibited by the law. What I mean is that if you feel like you may gain some advantage from it, do it; just beware of the risks that may come.
The main reason for not taking breaks, in my opinion, is that if you get used to it being normal, it becomes extremely easy to extend the pauses more and more every time. So yes, it's basically what you called "losing momentum".

That said, I've found that after a forced break (i.e. I'm on vacation somewhere or stuff like that), my performance with the kanji and the sentences increases quite a bit.

I had a couple of forced breaks, and I was always amazed at how quickly the orange cards piled up. So I don't recommend it not reviewing, liberating though it was to just fail a bunch of cards.

But I took several breaks from adding new cards, and it was both good & necessary. Okay, that's not entirely true: I went down to adding just one or two cards a day. Then the quality of my stories improved & I was still making progress, if s-l-o-w-l-y. Then after I felt better I had a couple of 50 card days. It averages out.

Momentum only matters if you have it. Most marathon runners don't take a break mid-race, but they've also done it before. Most of us hadn't.

I notice a similar pattern. When I learn a lot of Kanji quickly with a high recall rate, after some time that recall rate slows. I've found that spending a couple hours reviewing the last several hundred Kanji helps. Typically, I just head through each chapter, draw the Kanji and reaffirm the story. It doesn't take long to do 200-300 Kanji at all.

My retention rate jumps and stays high for a significant period afterwards.

Good luck.

Thanks for the replies. I'll keep going then; I only have roughly a month to go anyway.
Also, reviewing the last couple of chapters sounds like a good idea, I'll keep that in mind.

Depending on how long your break is, the schedule for the last stacks doesn't get that messed up, it's the first two that you will really forget. I echo the advice to review the last couple days' worth of kanji that you had learned before going into the reviews. On the other hand, you will find out very quickly which stories were really good and fail the rest to work on. Longer intervals, IF you remember the kanji still, can only help you. I'm forced to take many breaks and the hardest part is staying motivated, not relearning the kanji I always make sure to come and read people's motivational messages before getting restarted!

I took some breaks while I was going through RTK1, and although it is not recommended by the Surgeon General, it was ok.

The problem is just that it is harder to get back into doing it when I stopped. But on the other hand, a little breather from learning new kanji can help to freshen up.

Why not try to just reduce the number of kanji you're learning to say 1/4 of what you normally do? It should be refreshing, but you'll also keep your foot in the door, so you won't let what should be a 2-3 day break turn into a 2-3 week break .