[duxup]

Depends on your relationship with your employer.

My current employer, I'd tell them immediately, and even that I was interviewing.... but I have a great relationship with them and it's a small organization.

If it was a place I DID NOT have a great relationship with I'd give them two weeks notice, BUT also be prepared to be walked out right then and there when I told them (this was actually policy at some places I've worked).