[globba22]

I am responding from the perspective that this is in the US. If you are in a different country the customs may be different.

In my opinion, never, ever tell your manager or colleagues you are leaving until you are actually giving notice. It is an unfortunate nature of the modern working world that you have to sneak around to interview, etc, but that is the way it is.

It is bad for morale to tell your colleagues, and telling your manager you are planning to leave comes across as a vague and non-direct way to ask for improvements.

You may choose to give more than the customary two weeks, but surprisingly two weeks is always enough time to transfer crucial responsibilities. Shorter than two weeks comes across as unprofessional, and longer than that always seems to be too much time.