Sometimes it will only be 2 weeks and you want everything lined up before leaving. Other times, you gain more good will by giving more notice.

I worked for a small company previously, and my total compensation with them was driven my the contracts they had. There simply wasn't room for them to pay me more. They completely understood that I needed to do what was best for me and my family. I kept them in the loop from the time I started having serious conversations with another company. Because the company was so small, they had to change their bid strategy for future contracts with me leaving. They very much appreciated early warning. That's probably an outlier case, but a one-size-fits-all approach doesn't work for everyone.