Amazon starts to charge you extra anytime you exceed restoring 5% of the data.
If for example you save all your tax-related documents in Glacier, then you are audited then the accounts department or the government will want all the information. Not 5% of it. Not 10% of it. Everything. At that point Amazon will have you over a barrel, because getting the data out at a reasonable time frame will cost exponentially more than dripping out the data over the course of 20 months.