How do you know its the right time to leave a company?

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Lately, I've been feeling like I should move on to other opportunities, but I can't quite figure out if now is a good time, or if I should wait. Should I make a list of the reasons why? Interview at other companies before I leave? Give my current company plenty of notice before my hypothetical departure? How do I know that it's the right time to leave at all? I'd love to hear any opinion and guidance on this!