Assuming the page you linked is trustworthy they were established 1,600 years ago and two of them were alive in the 18th century so lowballing it they had to have lived for at least 1,300 years. So bare minimum they died sometime in the 1700s. If I'm being generous I'd say they died in the early to mid 1800s at the latest, assuming they're actually dead. Reason being that if LWA is set in modern times and not a century or more in the future their "deaths" probably weren't recent based on how they're treated as figures of legend.
Diana's ancestor kind of makes it hard to date though because as the founder of the family, her dying less than 200 years before the series seems odd to me.
Edited by Chariot on Mar 11th 2020 at 1:16:56 PM
Beatrix might have died longer ago - she might not have been one of the two who participated in rebuilding the school.
Tentatively, I said that Woodward died ~500 years ago, but stayed active as a spirit, so we have grimoires she penned centuries after her death.
EDIT: Also, thank you for responding to the ancient call!
Edited by Durazno on Mar 11th 2020 at 12:22:10 PM
In case anyone's still watching the thread, I'm finally posting the Big Silly LWA Fic.
On ff.net, and on Ao3. My apologies if the necromancy is in poor form - I'm just excited.
https://twitter.com/happylotl/status/1581485502288404480
So the show got a recent shout out in The Owl House.
Phoebe and Her Unicorn thread: https://tvtropes.org/pmwiki/posts.php?discussion=16104243270A34144300
Question, for anyone who still has this on their thread watch: how long do you suppose the Nine Olde Witches lived, and when do you think they "died?" (If we assume Woodward and Beatrix are gh-gh-ghosts.)
In a story I'm working on, I have a throwaway line where a character is confused because Woodward is implied to have designed a ritual after her death, and I have no idea what number to put on "Didn't she die ____ years ago?"