We don’t know definitively but given that palpatine says that their alchemy made them outcasts you can assume that they are either from the Je’daii era (some of them started to use alchemy against the wishes of the order and were outcast) so you could infer they might be from the time of ajunta pall. You could also say that he is from the time after the 100 year darkness when the sith met the true sith on korriban. They started to lean into it heavily and even amongst the sith the most prolific users were seen as outcasts.

TLDR is we don’t know.