It's a gradual progression, I think. The first chapters are really fast; the later ones can take a very long time, partly because they require so many more goods, so much more space, etc. I think I made it to chapter 3 within a few weeks, when I first started. But Halflings took me about 6 weeks, and I believe the one I'm in now, Elementals, will take longer. One hint that I wish I would have known when I was in an early chapter ... do not disenchant your portal profits. Keep them. You'll need them later to speed up research. I wish I had all the pps I disenchanted in the beginning, sigh.