I'm going to go ahead an say you should ask this in /r/asklinguistics, it may be too specific a question here.

My assumption is, you can probably read Shakespeare so 16th century at the latest. The best way to do this would be to take original documents (not translated) from as far back as you can, and see if you can understand them, then add a grain of salt, since this means you can communicate only with the nobility/educated.