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

What follows is a slight reworking of an answer I gave to a similar question a few months ago:

The answer is 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 archaic pronunciation, shifting meanings, and evolving figures of speech but you'd be able to recognize what you were hearing 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 contemporary English in structure, but the spelling was erratic and the dialect of the time would sound strange in some modern ears. Shakespeare, who was writing from 1589 to 1614, is often 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 (the centuries after the Norman conquest of England).

This is from Beowulf in Old English. This is what you'd hear in the 700s - 1000s (the centuries before the Norman conquest of England). Good luck with that.

The three examples I use are all poetic in nature. They are examples of heightened speech and nobody went out on the streets in those periods speaking in rhymed couplets or iambic pentameter. The day-to-day language was much less formal and it evolved from one generation to the next. What these examples do give us is a snapshot of the pronunciations and phrasings of the period. Informal conversations would have sounded less stylized.