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 pronunciation, multiple meanings, and figures of speech but you'd be able to recognize what you heard 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 Modern English in structure, but the spelling was erratic and the dialect of the time would sound strange in modern ears. Shakespeare is 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.

This is from Beowulf in Old English. This is what you'd hear in the 700s - 1000s. Good luck with that.

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If this question is already posted; I'm from germany, a little bit southern of Frankfurt, in fact. As germany wasn't united for that long, I guess it would be way harder to understand anything. What would my deadline be?

Would it be comparable to any modern day accents? I've heard different versions of either Geordie or Yorkshire being the original 'English accent'.

Something odd happened around the time of the Napoleonic Wars 1780s to 1820s when English really became the language we use today. I'd hazzard a guess that the proliferation of printed material (books) at this time had something to do with it - but prior to around 1780 English, at least in its written form, was quite different to today's prose, whilst anything written from about 1800 would be 99.9% indisitinguishable from modern writing.