weeks. In the roles I hire, there is generally 8 to 16 weeks between the day we find out we need to replace someone, and the day we have someone in the seat. Part of this is the hiring process, part of this is that the new person also will generally be giving notice somewhere else.

So, at least in my industry, it is unrealistic to expect to keep someone until you have a replacement, so the goal should be to enable a short-term transition to the current team. Personally, outside of executive positions, I think the expectation of anything more than two weeks isn't fair to the employees. YMMV