Sometime right before the Reformation.
So then everything the Church taught and practice up until then was correct?
Of course not, neglect of the Word of God was rampant, man made doctrines abounded. This set the table for the efforts of Leo X. That's why no one batted an eye at the simony. No one considered it odd that they now paid for indulgences or for masses that freed their loved ones from 'purgatory'. None were astounded at the use of secular power by the church. The common pew sitter or stander depending where you were, having no access to the Word on account of lack of Scripture in reach of their local church or because of illiteracy, relied on the same wolves who consumed their substance, to tell them what to believe. Then as now, secular power is only interested in retaining that power, dependency on gov't and ignorance of the goings on beyond the threshold of one's hovel were things the secular power and the church of the time wanted to encourage.
How long before? Is the date that important? It certainly was during the time that the church was engaging in the accumulation of secular power by promising spiritual succor to the kings of the earth that came to vie for its favor. Having relegated its mission from God to a secondary status, anything became good, secular power enabled them to reach out quicker to those it was charged with serving and had the added benefit of being more profitable.
Check your church's history and look for the beginnings of when it began to morph from a servant to all to something that desired to be the Master of all. There will be the time of the changing of the focus and reveal when the change that culminated in the 'Great Apostasy' occurred. When the real Great Apostasy spoken of in the Bible comes, it will dwarf what had come before it, but the cause and the motives will be the same. The opponent hasn't changed.