When does a reminder become too repetitive? For me, it involved repeating a fact three times within 500 words... made worse by too-obvious pointing at the thing through dialogue or in the narrative (see: Thank you, Captain Obvious)... or by ruminating on the same thing for a page every chapter... this isn't a hard and fast rule, of course. The smaller the fact, the easier to repeat without looking like it's being harped on. But you can go overboard with repeating even the smallest fact -- single words, as in Subtle Thing #3.
The other end of this problem is that you do need to repeat things that you want your readers to remember. It could be something as simple as a character's hair color, or a unique vocabulary word for your world, or an important bit of backstory. If it's been a long time (what constitutes a long time...?) since the readers have seen a character, reminding them what s/he looks like may be in order. The same goes for facts, vocab words, etc.
Writing this post gave me a chance to check on a sneaking suspicion I had: that I talk about what my main character's heart is doing. A lot. It's quavering, it's aching, it's sinking, it's tying itself in knots. Yes, it's a romance story and the heart is an easy stand-in for strong feelings, but anything can be taken too far.
An example of necessary, repetitive reminders for my readers, from Disciple: Saint Qadeem. He's seen once, in Part I, but that's very brief. The readers meet him for a slightly longer passage in Part II. We spend a whole chapter talking to him in Part III, but then he disappears again. It isn't until Part IV that he becomes a real presence in the story.
Saint Qadeem gets a physical description each of the times we meet him, in I - IV, and it trails off after that. I'm a sketcher, when it comes to physical descriptions, so these aren't long and they tend to emphasize what's relevant at the time. When we first meet him, in Part I, the most important things I wanted to get across were 1. he's ethnically quite different from everyone, 2. this is unnerving to a young girl like Kate, but 3. he's non-threatening.
Later, Kate is more openly curious about Saint Qadeem for a variety of reasons, and the most thorough descriptions come when she gets a chance to "study" him a bit. I hope that after fixing him in the reader's mind in Part III and IV, only slight reminders are needed.
How much repetition is too much for you?