Hey there! You might get some more info in this thread, but while you're waiting you can read through some other related threads. This section of the wiki has a link to all of the threads: http://www.reddit.com/r/AskHistorians/wiki/language#wiki_how_far_back_could_i_go_and_still_communicate.3F

/u/texpeare covered it well.

Probably some time in the early 1500s, though the late 1400s might be possible with some patience and concentration. There might be some head-scratching over pronunciation, multiple meanings, and figures of speech but you'd be able to recognize what you heard as English. The Battle of Bosworth in 1485 marks the beginning of the Tudor Dynasty. That same year saw the first English publication of Le Morte d'Arthur, often cited as among the earliest examples of what we now call Early Modern English. EME is very similar to Modern English in structure, but the spelling was erratic and the dialect of the time would sound strange in modern ears. Shakespeare is the textbook example of Early Modern English.

This is Act 3, Scene 1 of Shakespeare's Henry V recreated in Original Pronunciation. If you understood what you just heard, you'd do ok in the late 1500s - early 1600s.

Here's a reading from The Canterbury Tales in Middle English. If you can follow that without difficulty, you might be ok in the 1100s - 1400s.

This is from Beowulf in Old English. This is what you'd hear in the 700s - 1000s. Good luck with that.