This depends on your language skills and your knowledge of other languages. Your biggest problem would likely be the vowel changes that English has gone through. The second problem would be the many many loanwords that English has taken from other languages.
First a quick look at the history of the language. English has started out as a West Germanic dialect, like Dutch or Frisian, part of the larger family of Germanic languages, like German or Swedish. The first phase of the language is what we call Old English. This came into being around 450/500 A.D. and was a proper Germanic language, and as such, most current day English people have trouble understanding any of it. An example of an Old English piece of text would be the following:
” Hwæt! Wé Gárdena in géardagum / þéodcyninga þrym gefrúnon**·** / hú ðá æþelingas ellen fremedon” (Start of the Beowulf; read more with translation here. And here you can hear what Old English sounded like). Grammatically, the language is more synthetic, meaning that it uses inflection (like cases) to express grammatical categories.
Oh yeah and sometime during this period, the Vikings came by and the Danes and Norse settled in Britain as well, bringing some of their language with them. While they spoke another Germanic language, it was different from West Germanic and as such, it had a distinct influence. Some words you might not think are Norse: dirt, egg, glitter!
In 1066, the Normans came over to the island to conquer it, and they brought the French with them. Anglo-Norman was the language of the upper classes for a good long while, and it influenced English heavily (some people say up to the point that it became a pidgin language). The grammar simplified and the language became a bit more analytical, meaning that some of the inflection disappeared. As you may guess, if you don’t have inflection to express grammatical categories, you need something else.
Analytic languages use word order to express the same meaning. For instance, in English ‘The dog chases the cat’ means something else than ‘the cat chases the dog’. However in Latin, which was a more synthetic language, the inflection makes the difference in meaning: ‘canis fugat cattum’ and ‘canem fugat cattus’ mean the opposite, despite being in the same order. Here is an example of what Middle English sounded like (the beginning of the Canterbury Tales).
Pronunciation may have changed a little, but not as much as the grammar and the loanwords did. However, then the 16th century rolled around, and (Early) Modern English rolled into being. Between these two types of English, the most important vowel shift took place; the one that made sure English students across the world can’t make head nor tail of the English spelling – because English spelling didn’t actually change all that much whereas the way the words were pronounced, did.
It’s a very long story to get into all the details of the Great Vowel Shift, but the most important thing to remember is that basically all of the long vowels moved upwards in the mouth, and two became diphthongs. Before the vowel shift, the vowels were pronounced more or less like in most other European languages (think Italian or German). After, you got, well – this. Also, more loanwords were added, from Latin, but also from German, Dutch, and more French.
As you can see, how well you could understand people, more or less depends on how good you are at dealing with sound changes (for instance, do you find it easy or difficult to understand the many different accents and dialects in English?) and whether or not you speak any foreign languages (French and Dutch/Frisian/German are your best bets for Middle and Old English, respectively). In general, we would say that most Americans without any linguistic knowledge could probably understand a fair amount from the 17th century onward. If you have no trouble with accents, >1500 would probably work for you.
2. Barbara Fennell’s ‘A History Of English’