The Present Perfect is connected to the present and may refer to a completed action. However, we are not interested in the time of the action, rather in the results of that action as they affect us now. That is why the tense is called the Present Perfect.
The Past Simple is not connected to the present because the action began and ended in the past. Specific time expressions are used with the Past Simple, like: ago, yesterday, last week, when (when I was a child), etc.
See these examples:
I have broken my leg. (I can't go dancing.)
I broke my leg last summer. (finished past action).
So... it would be better to say instead of
"It's a long time since she has been off work"
It's been a long time since she stopped working.
(It's = It has)