hi! fyi, you can get started on some previous responses in the FAQ (link on sidebar)
I'll take a stab at this, but I must admit I am going off of an audio lecture series I listened to recently about the history of the English language. http://www.goodreads.com/book/show/2523660.History_of_the_English_Language
You can't really put an exact date on it as language is very fluid. Things change over time. That being said, probably the point where things would start to get confusing would be around the Great Vowel Shift, which occurred over a rough 100-150 year period in the 15th and 16th centuries. So if you were to time travel back to the year 1400 you would have much more difficulty understanding English than if you were to time travel to the year 1600. The further back you go the less understandable English gets. In fact, if you go back before the conquest of 1066 it would be easy to mistake English for Dutch or Danish. Some linguists speculate that if it were not for the Norman conquest, English and Dutch would be mutually intelligible languages today. Anyway, I hope a linguist with more expertise can chime in, I am also fascinated by the history of languages.