Announcing the departure in 3, 2, when?

December 31st.

This is the final date of employment at my current church.
It was announced to the church a month ago.

And I've known my final dated for a while before then. Maybe not officially, but the funding was always finite.
The church? For some, the first they heard about my departure was when it was announced.

So how much time should a church have before a ministry agent leaves?

As one who has been on both ends of a lengthy and swift departure window, both have positives and negetives.

But in reality only a few things matter...

What are the circumstances of the departure? Was it amicable? 
Who needs the departure date more, the church or the departing worker?
Is the person going to another placement?Does the church need adequate time to arrange another appointment?
Is there a viable succession plan in place for things to continue?