I answered a related question a couple of years ago, and I would say that up to the late XV century you should not have that many problems understanding people. I hope it helps.

https://www.reddit.com/r/AskHistorians/comments/7x4dyy/could_a_modern_day_spanish_speaker_have_a/

If you go further back in time, the differences would become more evident, but even XIV century Spanish can be understandable. I will give you an extract from a text from the time, just to be sure how well you can understand it:

Asi como ha muy grant plazer el que faze alguna buena obra, sennaladamente si toma gente tan bajo el, a faz quando sabe que aquella su obra es muy loada, et se paga della mucho las gentes, bien asi ha muy grant pesar et muy grant enojo quando alguno a sabiendas, o aun por yerro, faze o dize alguna cosa por que aquella obra no sea tan preciada o alabada como deuia ser. Et por probar aquesto, porne aqui una cosa que acaeçio a un cauallero en Perpinnan en tiempo del primero rrey don Jaymes de Mallorcas.