GtW - (non-medically trained comments follow!) from personal experience (mostly as commercial pax), I would estimate that from top of descent @ cabin altitude of 8 - 10k ft, I have to keep the eustacean tubes clear the whole way down. If I get a pressure imbalance 'locked in' which I fail to clear within a minute or so, then it gets progressively more difficult to clear. The point at which I am finding the ears most prone to pressure imbalance is when I estimate cabin alt is between 5k ft and 2k ft. Generally speaking, it is very rare for me to have any problems clearing my ears from 2k ft down.