From the Early Foundation Establishment stage to the intermediate stage, it might take three or four years. From intermediate to Advanced, four or five years. From Advanced to the Foundation Establishment Great Circle, another four or five years, and then another four or five years to reach the Golden Core stage.
And this was still the time required for a single spirit root. If one had multiple spirit roots, the time and effort required would double!
As for the time needed for each level gained after reaching the Golden Core stage, that would be even more.
Chapter 38