One answer to this question can be found in the catalogue of libraries of Henry of Kirkstede, written in the 14th century, up to his death in the 1370’s. Henry was a learned man, and for a while was the prior of the abbey of Bury St Edmunds, a wealthy priory which is well known now for the poor and sometimes violent relations between the priory and townspeople. His catalogue covers books held in religious houses in Bury and a number of other towns and country sites, so whilst it does not cover a large area, it does give an idea of what was thought important by all these learned men.
Anyway, what he wrote in entry 259 was:

HERMES qui et Trismegistus sive Mercurius floruit tempore Phillippi regis Macedonum A Mdi 3500 et scripsit De verbo perfecto et De Mathesi. Vide in litera T verbo Trismegistus.

So, someone called Hermes Trismegistus was around in the time of the father of Alexander the Great. That is, around 340’s BC.

There is another entry for Hermes, under “Tremegistus vel Trismegisus qui et Mercurius vel Hermes floruit a.m. tempore <blank> et scripsit juxta Vincentinum in Speculo historiali lib 5to.”

He then mentions “Ad Asclepium lib” and “De verbo perfecto lib 1.” and some others, not specifically alchemical.

From both these entries it appears that the more classical Hermetic knowledge had been written about. Not the alchemical stuff.

There is an entry too for Asclepius, the god of medicine in ancient Greek religion and mythology, and a narrator or recorder of much of the corpus Hermeticum.

Just to add to the confusion, Holmyard in his “Alchemy”, points out that there were 3 Hermes known of to the Arabic alchemists, one pre-flood, one not long after and a third in a time that I suspect is more that of Philip of Macedon. Thus Henry or the authors he was copying weren’t very well versed in Hermeticism. Which isn’t very surprising, given their comparative lack of information access.

By contrast, the English translation of John of Rupescissa’s Book of the Quintessence, therefore what John thought about Hermes in the mid-14th century, Hermes had the secret of the quintessence from an angel after Noah’s flood. Which is of course a lot earlier than the 4th century BC.

So I expect that a wider search would find a number of different dates of Hermes, probably about those that Holmyard suggests.

Having scanned through some of the translations of alchemical works that I have, it does surprise me how little Hermes comes up. E.g. in the Scala Philosophorum of Guido de Montanor, he is hardly mentioned at all, I saw more mention of Avicenna and Albertus Magnus and others. Of course as a pagan, he wasn’t exactly the best person to bring up in a discussion about how the world worked, but what it also shows is how alchemy had accumulated a long intellectual history of it’s own, 12 and 13 centuries of it before even Henry of Kirkstede. Thus medieval authors are happy bringing up pseudo-Democritus and the various arabic authors, and Hermes, despite being the most ancient and author of the Emerald Tablet (Not really, but that’s the story) just does not come up so much.

The new question that occurred to me was, can I use the library catalogues to find out more about the presence of alchemical books in Medieval England? Which got me enough information for another post, coming soon.