It really depends.
If it was a quick Q and has been answered and it's only 3 days old... don't
bring it up. It's been solved. Any "new" information should be put into a new, or more appropriate, thread. That goes for any "solved" thread. If it is solved, you don't need to "hi-jack" the thread... just make your own! It's considered "dead" when it's done.
If it's a thread like a memorial thread, a "fun" thread, etc that has already had replies, never
bring those up. I give a week's rule on those ones, personally. Especially memorials! Is there already replies, support, etc? Ok, let it be.. If you reaaally feel you have input, message the person directly. If you bring it up, everyone will start commenting, and it's totally unnecessary. "Fun" threads like picture threads shouldn't be brought up after a week unless there weren't any replies, in which case the creator might like a few comments! If they've already been commented on and have given "satisfaction" to the owner... leave it. There are new threads that need attention. Again, if you want to make a comment, contact the person directly. There's no need to push down other threads just because you have an input. Disease threads I use this for as well... There are SO many more emergencies, no need to push them down! Just message the person and ask if they're still having problems so all the other emergencies get attention.
2 weeks can be applied to "hot" threads, very active threads, informative threads, etc... I hope you know what types of threads I'm referring to! As a rule of thumb, only bring up threads you find will be beneficial to other members and the original poster.
Is it just for you? Do you want satisfaction? Is your new post directed towards yourself? Etc? Then don't comment! Is bringing this thread up an interesting, helpful thread that other members will appreciate? Were there no comments and it's been under 2 weeks? Etc? Then comment!
I hope this clears it up for you... But never
comment if it's been "dead" over 2 weeks!
Mods/admins can't close every single thread
that has "died". First of all, they can't keep track of all of them, and secondly, if they've been inactive threads for 2 weeks, then they are far buried and it'd be difficult to dig them up and lock them. Take the responsibility and watch over yourself! It's easy to check the dates.
Any more Q's?