OK, let me say this: perhaps I gave an answer above the quesiton; meaning, an answer for 3 vs 4 as opposed to 2 vs 3. I will admit that 2 vs 3 can and probably should be learned by the notation already provided. However! A player cannot play every 2 vs 3 section by thinking consciously, "OK, the second note of the 2 part must go halfway through the triplet set, which puts it 1/6 of the set past the 2nd note of the 3 part...." or however it goes. This leads to robotic sounding playing instead of smooth, flowing triplets. By all means, if you need to learn how the rhythm works by looking at a diagram, analyzing it, etc., go ahead and do it. I still think that after this work is accomplished, the student needs to sit at the piano and play these over and over again until they flow, and s/he can play them without thinking about them.
Nightmare, let me say that, for the 3 vs 4 of the Fantasie-Impromptu, you're exactly right: one day, it will click. It will take a while, but it will. For me, it was nearly two months. I'm not sure how that relates to the average, but it was two months of playing nothing other than the first two measures after the right hand comes in. It was hard, seemingly impossible, and my mind repeatedly shut down and simply refused to work on it any more. I came close to quitting several times, but stuck with it. My teacher kept telling me interesting ways to count it out loud, or ways to map out the rhythms, but it just didn't work for me. I couldn't translate numbers on a sheet into smooth, flowing rhythms on the piano. But, after those two months of tapping it out on the piano, on the piano bench, on restaurant tables, on my lap, etc., it clicked. I don't know what I did to make it click, but it clicked. After that, I had the song learned and memorized in a month. (The notes aren't nearly as hard as they may look; lots of patterns, repetitions, variations, etc.)
Oh, and xvimbi, when I said hours and hours, I meant for the Fant-Imp.; I don't think it's possible (for most people at least) to learn 3 vs 4 in 20 minutes. I don't remember exactly how long it took me to learn 2 vs 3, but I'm almost positive I learned it in one lesson and had it perfected by the next week. Let me finally explain that by no math, I mean no dividing and finding fractions, etc. I've always loathed fractions anyway.

Good luck Nightmare and others
Terry