One way to handle things is to put (spoilers) in the thread's title if you (the thread creator) feels there will most likely be spoilers in that thread. Then if anyone who reads that thread does read anything that's a spoiler, then that person is at fault because there is a warning.
But (IMHO), I've not read every Star Trek book out there. In fact, it's not that difficult to miss out on say the eBook only releases if you don't normally read eBooks. So I do feel that six months is not nearly long enough.
Here's a very good example of why spoilers out in the wild are not good. We have Star Trek: TNG: Cold Equations Book 1: The Persistence of Memory
. It's a sequel to Star Trek: TNG: Immortal Coil
. Yes, IC was out more then a year before TPoM. But it's quite possible not to have read IC and because of TPoM, now IC will be read by a number of people. Spoilers for IC would not be a good idea just because of its age.
So please, it's very little effort to use spoiler codes. So please do so regardless of the book's age.