In Italy we all study Dante, Petrarca and Boccaccio in school, which are 1300, and it's quite easy to understand them beyond some unusual words. 1200 poetry is easy enough too.

There's not much literature older than that, cause people preferred to write in Latin, the oldest bit in "volgare" is the Indovinello Veronese[0] which is from the 8th or 9th century and at that point it's almost latin spelling-wise, it's understandable if you're well educated but wouldn't be understandable by everyone.

[0] https://en.wikipedia.org/wiki/Veronese_Riddle