I find that improvements comes tend to come in jumps. I went from sup-30 to about sub-26 in what seemed like a week. I steadily improved, slowly, and then jumped from ~21-22 to ~19 in a couple days. I haven't had a "barrier" yet, but I have noticed that there are periods of relatively slow progress, followed by short periods of very rapid progress. Those days of rapid progress feel amazing, but usually they're followed by frustration when I wish I was still improving at the same rate. My guess, most people follow the same progression patterns. When these jumps occur probably differ among cubers, but I'm sure the pattern is there.
On a side note, whenever I see something like "I was sub-XX in XX months", it's generally exaggerated. I've checked people's youtube pages, and their competition records and found they were cubing long before they said they were, or actually improved slower than they claimed. I always take those comments with a grain of salt. Not implying at all that anyone on this thread is doing that, but something I always keep in mind when it comes to "well I'm better than you" posts (which there seem to be more of lately).