Mad_Jack
Legend
I think the definitive answer to the question is that a thread is necroed when people see it pop up again and go, "Oh, hell, not this again?!?!"...
Whether that's a day, a week, a year or a decade doesn't make much difference.
Reopening an interesting discussion after sufficient time has passed to generate new and/or different perspectives on the topic isn't a necro.

Reopening an interesting discussion after sufficient time has passed to generate new and/or different perspectives on the topic isn't a necro.