You may be interested in this section of the FAQ. This question is difficult to answer, simply because it's impossible to actually do so. Additionally, people do have varying ability to understand different dialects of English today. This'll also depend on what historical dialect you're listening to, and what you speak now. If you're an American, understanding someone from a few hundred years ago in your hometown might be easier than someone from rural Northumberland now, let alone in the past.

Have a look at this video of actors pronouncing Shakespeare as it was originally pronounced. That's early modern English. The actual reading is from 2:55. Keep in mind that when you hear it, you've just heard the modern reading, and that it's reading, not natural conversation. Much older than that, you start to hit the earlier stages of the Great vowel shift, which makes comprehension rougher. Here, for instance is Middle English, and it's much tougher. When written, Middle English is usually translated, and because of the history of English spelling it's more dissimilar in speech.