Oh good catch! Daisy does mention they haven't had a human visitor in years and she says Eleanor lived in Oakridge for over 10 years.
Bartelby says she arrived about ten years ago.
So if we were to take the vague descriptions of timing into account I guess it's safe to say, she arrived over 10 years ago (probably 11-12 yrs ago) stayed for about 10 years and then just up and disappeared.
I have a vague feeling that she might have passed since no one brings it up. And also why MC might've been so confident about it.