I subscribe to the AtariAge take on necrobumping- if you've got something relevant to add that warrants bumping a thread, how old it is isn't terribly important. Of course, it's something that varies pretty wildly around the net.
Personally, I think for general opinion kind of things ('What's your top 10 games', 'is X system worth buying', etc.) maybe keep it within about a year. For something where the dialog has changed, older can be fine- for example, a thread about Limited Run Games from back at Breach & Clear's release that hasn't been active for a year and a half could reasonably be bumped to discuss adding Switch games to the lineup on a preorder basis- since now there's new information that might make people want to revise their earlier statements.
I'd love to hear the mod's take on this, since it's kind of a divisive thing.