If you're still interested, this is a really common question on AskHistorians; take a look at these similar questions and their responses:

How far back in time could you go, before modern English becomes unintelligible to inhabitants of the British Isles? by /u/Bayoris

As an English speaking Englishman... how far back in time would I have to go before communication would start to become a serious problem? by /u/texpeare

and Could a modern english speak with a person from the medieval in england? by /u/cdesmoulins, which has additional links to past answers