I have really struggled to pinpoint what time period Lefay is from. She mentions Avalon and being persecuted for her ears and people thought she was a demon or a witch, but she knows of Japan and mentions Britain and Japan being friends. The English did first contact Japan as early at the 16th century, but well established trade and diplomatic relations didn't come until after the Meiji Restoration in the mid-19th century and I suspect she was talking about the Anglo-Japanese Alliance which was signed in 1902. She also talks about in the past tense (which could be a translation error) and the alliance was dissolved after World War I in compliance with postwar treaties. She also mentions being British so she's at least from a period after the Act of Union of 1707 which merged England and Scotland into Great Britain. I think it's unlikely people would persecute her as a witch too late into the modern era, however I suspect she is from the 20th century, possibly the 1920s.
Edit: She mentions "Old Johnny's Book" in another chapter which I think refers to J R R Tolkien (John Ronald Reuel Tolkien) and his works the Hobbit (1937) and Lord of the Rings (mid-1950s) so she might be from from a more modern period than I originally thought.
I am probably thinking way too hard about this, but it's been a fun exercise for me at least.