I don't know what you mean by "a regular knowledge", nor why you are taking a Latin I textbook with you, but assuming you had a classical pronunciation scheme and could speak and write conversational Latin, I think you could get back to the 2nd or 3rd centuries BC. Classical writers in the 1st century BC show an awareness of "early latin" - words, spelling, and phraseology that seems "antique" to them, but it never presents them with any problems of understanding. But Polybius does mention reading a treaty from 452 BC that was quite difficult for him to understand, so I think much past the 300s and you will run into real difficulty.
This has actually been asked and answered quite well at least once before.
One of the points which has not been brought up by the other two excellent answers is that it's also not a direct line back which correlates with time. You would have a much more difficult time being understood in the 6-8th c.s than you would in classical Rome.