In this previous thread, I linked to a transcription, translation and audio recording of the Peterborough Chronicle, written during the mid-twelfth century in a transitional period between Old and Middle English, and you can see for yourself how much of it you can understand.

There is also a link there to the relevant section of the FAQ.

By the time we are in the heyday of Middle English, in the 14th century, most modern English speakers can, with some difficulty, generally read their English. (For this you can have a look at Chaucer or William Langland for prominent examples.)