Has the post not been responded to in months, and you're coming in to say something like "same problem here"? That's a necro bump.
Has the post not been responded to in years, and you come in with actual information which can revive a discussion or contribute in a meaningful way? Not a necro bump (generally).
It's not a binary issue of time. There's other factors that can play into it as well. If you're making an actual contribution to an OLD discussion, I and much of the rest of the staff are happy to let that stay, but if you're just leaving what equates to a "+1" comment, not cool, chief.
Edit - but, if you wanted an idea of when it starts getting into necro territory time-wise? idk, probably 4-6+ months, probably. Like I said, not quite a binary determination, there's no hard cutoff there.