It really does depend on the job and your relationship with your employers. If your boss is a friend, you may want to let him/her know earlier rather than later. When I quit the first time, I gave about two months notice because I had to hire a replacement and that took time. Plus, my boss was supportive. On my third hike, I gave notice several months early because there were changes going on in the office, and I had to explain why I wasn’t going to be a part of those changes. No problem. One thing that can happen in some offices - talking about leaving can mean that they fire you on the spot - so don’t say that you are leaving until you can actually afford to be without a job. Alternately, they may take away all responsibility, so you are left with nothing to do. This can make the last weeks very very boring.
On my current job - I would love to let them know that I am leaving, but I am somewhat afraid of the response. My main boss is mercurial - he could wish me well, he could pitch a fit. Chances are, he’ll do both. I’ll give two weeks, maybe three.
One thing I learned the hard way - if at all possible, give yourself some cushion time between leaving your job and getting on the trail. There are always last minute things to do, especially if you are packing up a house or apartment. Before the CDT we worked all day Friday, finished emptying the house at 3:00 a.m., then caught a 7:30 plane that morning. It took a week to get over the total exhaustion we felt. This time we’re giving ourselves three weeks to finish packing and do a training hike of a week to ten days. It should be a lot less hectic.
Ginny