I had a hard time deciding between 6-months and use-your-own-discretion, but I came down on the discretion side because I feel that 6 months is just too short a period - I'd probably have an easier time supporting a 12-month rule.
That having been said, I do think having a 6-month rule is much preferable to having a "code all spoilers" or "code no spoilers" rule.
Whenever I post about something, I probably tend to err on the side of caution and spoiler-code things that others may feel don't need them. But I don't expect everyone to follow my way and I wouldn't get upset if I accidentally got spoiled by reading something about a book that is, say, 5 or 10 years old that I just don't happen to have read yet. I've read just about everything published since the "A Time To..." series except for Vanguard and NF, but there's a lot of stuff from about 1998-2004 that I haven't read yet (working on it, though). I understand that coming to this forum carries the potential of accidential spoilage and I'm prepared to accept that risk.
I do definitely support the idea of blanket spoiler warnings for threads involving new books and I feel entering those threads before you've read the book should be at your own risk. I will visit those threads before the book is actually published to see the artwork and see how others feel about the book, but once it is actually out I stay away from the thread until I've read the book. It's worked pretty well for me so far.