When was scripture alone sufficient?
TLDR; Never. Scripture has never been sufficient, and never will be.
Your question is based on a false premise. The irony is that you actually point out the fallacy in the opening sentence of your question (emphasis mine):
Sola Scriptura is defined as "the Protestant Christian doctrine that the Bible contains all knowledge necessary for salvation and holiness."
What is necessary for salvation and holiness never has been, nor never will be scripture. Rather it is knowledge (precisely what knowledge is necessary for salvation and holiness is subject to debate, and not directly relevant to this question).
How this is interpreted by Protestants is that if someone completely unfamiliar with Christianity were handed a Bible and read it, they would have enough information that if they chose (or according to some doctrines, if they were elected), they could make a soul-saving decision to follow Christ.
In other words, they would not need to read the Bible and then also start attending church to gain more information. Or they would not have to read the Bible and then also attend a catechism class, or learn from Catholic Tradition, or also read the Book of Mormon, or also read Joyce Meyer, or also watch TBN, or anything else. That doesn't mean these other things might not be edifying or otherwise useful to a believer, but all the knwoledge that is necessary for salvation can be found in scripture.
That also does not mean that this information can be found only in scripture. It's possible (and in fact, quite common) that someone becomes a Christian without ever having read the Bible, by assimilating the necessary information second-hand (from someone who had read the Bible), or by another means of revelation.
The important part is that, to an adherent of Sola Scriptura, no other form of knowledge acquisition will contradict the scripture. If a missionary tells you "To be saved you must do XYZ," that XYZ will either be consistent, or inconsistent with scripture.
Asking "When was scripture alone sufficient?" is asking a nonsensical question, because it puts the emphasis on the scripture, rather than on the knowledge contained in the scripture.
If instead you wish to ask "When did the knowledge alone become sufficient?" the answer is, "It always was."
Scripture came to contain the knowledge which was always necessary and sufficient.
Perhaps you could ask, then, the related question: When did scripture come to contain the minimum of knowledge necessary and sufficient for salvation?
I believe most Christians would say that this information is contained in the Old Testament (if primarily through types and shadows), indeed even in the oldest form of the Old Testament, the Torah. So the answer to that question would be "The moment scripture began to exist as scripture" which was apparently roughly 445 BCE.