Should be "how far back in time can you read English?" The language itself is what is spoken and the writing, while obviously related, is its own issue. Spelling is conventional and spelling and alphabet changes don't necessarily correspond to anything meaningful in the spoken language; meanwhile there can be large changes in pronunciation and comprehensibility that are masked by an orthography that doesn't reflect them.

Indeed, I remember being in Oxford in the 90s and an older man approached me and spoke to me in English and I couldn’t understand a word he said. My ex-wife, who’s an ESL speaker who speaks fluently and without an accent has trouble with English accents in general. Similarly, in Spanish, I find it’s generally easier for me to understand Spanish speakers than Mexican speakers even though I learned Mexican Spanish in school and it’s been my primary exposure to the language. Likewise, I generally have an easier time understanding South American speakers than Caribbean speakers and both sound little like Mexican Spanish. (The Spanish I understand most easily is the heavily accented Spanish of non-native Spanish speakers.)

Accents have diverged a lot over time and as I recall, American English (particularly the mid-Atlantic seaboard variety) is closer to what Shakespeare and his cohort spoke than the standard BBC accent employed in most contemporary Shakespeare productions).

Yeah it’s really just the glyphs that are changing here, and occasionally the spelling, otherwise the words themselves are still fairly recognizable if you’re well-read.