How long do you suggest a key should be valid for?

I'm not certain, I'd set an expiry on one I created with an open end value in
2016 to 2018 y/day but now I'm wondering if that's a wise move or not?

I say that as my limited understanding of keys so far is that they gain
greater trust when signed by others but if I expire a key after only less
than 12 months to go then surely I have to start all over again with getting
the new on signed etc. so in my mind it's a disincentive to expire it?

Thoughts welcome.

Paul

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