The real reason I don't particularly like this question is that it assumes all cubers to be about the same. Everyone improves differently, so even if getting stuck at 18 seconds is common, it doesn't mean you will get stuck at 18 seconds. Putting that aside, I've been stuck in the mid 13s for a while, and now again in the mid 11s. You just never know when you'll improve. As I said before, for me, most improvement doesn't come from getting more good solves but from getting fewer bad solves. What'll happen is my average will stay about the same for a while, but instead of getting a st.d. of, say, 0.7, it will go up to 1.2. In other words, I'll get the same average, but more good solves and bad solves instead of more normal solves. Then, the bad solves go away, my consistency increases, the good solves don't really go away, and I get better, the same way if the WCA did an Ao4 system where first the worst solve was eliminated, then the best and the next worst, and the middle two averaged, peoples' averages would be faster- {10, 11, 13, 14, 18} would be a 12 average and not a 12.67 average.