How much do you trust your manager?

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Note, the question is not “Do you trust your manager?” — thats a different theme altogether (•) — it is “How much do you trust your manager” (the presumption being that you do have a halfway decent manager).

Why the question? 
Let’s say you’re thinking of leaving your current job, for reasons unrelated to your company. Maybe you’re switching to a different job for much more money, or you want to take a year or two off, whatever. This could be in a few weeks, a few months, or maybe even “a year or two”.
Soooo, do you talk about this with your manager? (••)

The thing is, the moment you talk about this with your manager, you present them with a dilemma — they now know that you might be switching in the future. Which means that they need to start thinking about replacements, insurance, etc. Mind you, this isn’t about them being jerks — they could have great personal integrity, have your best interests at heart, etc. However, it’s not just you, they still need to think about the rest of the team, the projects being worked on, the product roadmap, marketing, etc., and this is all before even getting into stuff like their own career, bonuses, vacation schedules, etc.

The point, of course, is that your manager has responsibilities other than you. Not to say you aren’t important, it’s just that there are other important things too. My rule of thumb in this case is to not discuss this with your manager. If you’re the exception, and your boss is one of The Good Peoples, and a personal friend, then, maybe, go for it.
But even then, be very careful. If they are genuinely good at what they’ll do, they’ll work with you on the impact to others.

Let’s expand this scenario a wee bit though — imagine that you’ve actually decided that you’re going to leave (again, for reasons unrelated to your current company).
In this case, when do you talk about this with your manager?

As you might have guessed, it’s the same scenario as before, except that you’re forcing your boss’s hand. It’s no longer a “what if” thing, instead, it’s more of a “action needs to be taken” scenario. Now, they have to actually start dealing with the impact to the team, projects, roadmaps, etc.
Mind you, it doesn’t matter if this is, oh, 9 months down the road — remember, hiring takes time. What’s more, the timelines and budgets are no longer in your favor, e.g., you might want to leave after 9 months, but they want you to leave after 4…

The point here is that if your notice period is, oh, 4 weeks, then it’s ok (and good!) to give more than 4 weeks notice to your manager. But if you’re planning on leaving in 9 months, think long and hard about your relationship with your manager, the team, etc., and when you actually tell them that you’re leaving.
It all comes down to the same thing — how much do you trust your manager…

(•) For more on managers, great ones at that, and trust, read On Great Managers.
(••) Mind you, if your manager isn’t trustworthy (and you’re at the job because you need the money, or the experience, you don’t have a choice, etc.), then you definitely should not be discussing this with them — but you already know that, right?)