The norm is 2 weeks, because of that, I have typically always submitted them 3 weeks in advance.
My rational for doing this was to help train my replacement and get a great reference for having done so.
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I did submit a letter once though, where 2 of us resigned on 1 letter. We were the only 2 guys in one very important department! In doing so, we also unleashed hell on a manager that we despised and then walked out the day after she left on a 2 week vacation. Our "3 week" letter was submitted to her boss the day before she left and so she thought she was in good shape until she returned... NOT! We made a power play that cost her ([i]and other management[/i]) in a big way, and then we just started our new jobs immediately!
I don't suggest many people do what we did, but it worked out exactly as we had planned, and the entire "corrupt" management structure collapsed, making for a better work environment for the remaining employees! [-!-]