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It depends almost entirely on seniority and your relationship to the company. I found that for most IC jobs, once you give your two week notice you immediately lose a lot of the relational power that you previously enjoyed. There is no incentive to take on new projects, so you quickly find yourself out of things to do. Depending on how the company handles your departure, two weeks is either nowhere near enough time or way more than necessary. As you increase in seniority, there are more people who rely on you to get your job done. Therefore the task of finding a replacement and training them is larger, and so more lead time is appreciated. For manager level and above, a month would be ideal if you can swing it. For executive roles, you should ideally be letting your fellow executives or the board of directors know about your departure multiple months in advance.