If we're talking about England, I think no more than 50-80 years for level 1 (in some places 2). With level 2 maybe 100-150 years. With level 3 not (much) before Shakespeare (there was a linguistic change that made lots of words different). With level 4 we go much further, maybe 1300-1400.Before 1200-1000, probably the level would be 5. If you want a chronology of the English language, look at this Wikipedia page.It has some exemples that can make you have a better idea of the change in this language. Remember that the ability of understanding the other languages without knowing them depends a lot on the education recived and the personal inclination in languages. So your time traveller will have less problems if he's studied also a foreign language and knows some things about phonolgy and grammar and if he has studied some not-modern literature.

If we're talking about England, I think no more than 50-80 years for level 1 (in some places 2). With level 2 maybe 100-150 years. With level 3 not (much) before Shakespeare (there was a linguistic change that made lots of words different). With level 4 we go much further, maybe 1300-1400.Before 1200-1000, probably the level would be 5. If you want a chronology of the English language, look at this Wikipedia page.It has some examples that can make you have a better idea of the change in this language. Remember that the ability of understanding the other languages without knowing them depends a lot on the education received and the personal inclination in languages. So your time traveller will have less problems if he's studied also a foreign language and knows some things about phonology and grammar and if he has studied some not-modern literature.