If a sub has specific rules for what constitutes a spoiler, the period after which something can be considered no longer be a spoiler, or how to handle spoilers in general then of course I will follow those rules.

If a sub doesn't have such rules explicitly stated then in practice I'll mark anything as a spoiler that others might consider to be a spoiler out of respect. In time, as I learn the intricacies of a given sub, I'll adapt to the sub's unwritten ways and relax somewhat because let's face it, marking every little thing as a spoiler is a tremendous crotch-ache.

In principle, the way I'd prefer to handle the matter is to give potentially spoilery material a reasonable period, depending on the type of spoiler, during which time it's to be marked as a spoiler but after which is fair game.
For example, to my mind revelations about future casting, leaked episode scripts, papped set photos, and things of that nature should always be marked as spoilers. Anything directly referencing an aired episode should be marked as spoilers for a reasonable period after airing, say 48 hours or a week, whatever, but after which should be fair game for discussion without all the crap of adding spoiler tags and caveats and so forth.
This gives viewers who've seen the episode a way to respectfully discuss the goings-on while leaving viewers not yet caught up time to catch up without chancing across accidental spoilers. It's not, perhaps, the ideal scenario depending upon your own opinions regarding spoilers but I consider it the fairest way all around.
It does rely on the enlightened to be respectful towards the ignorant (unlikely) but it also places an onus on the ignorant to make the effort to keep up with a show (or book, or game, or album, or whatever) as it develops (not always possible, admittedly).
I can see that there're flaws in this reasoning but personally I find it highly naive and unrealistic for a given random behind the times to expect to visit a subreddit, forum, discussion board, or whatever, that's devoted entirely to a show and expect to never see spoilers. These are places that are active, devoted, and enthusiastic in the subject matter and expecting subscribers to become less active, less devoted, and less enthusiastic is unrealistic and entitled in my personal opinion.
Without wishing to digress too far, I consider it similar in concept to a person entering a public toilet and expecting it to not smell of human waste or to be able to have a phone conversation in the absence of the sound of people shitting or flushing, or to a person moving to live next to a pub offering live music and then complaining about the noise. We'd rightfully consider that unrealistic to expect yet many expect just the same from subreddits, forums, and the internet in general.
Frankly, if you've visiting any site devoted to a show you should expect things to be potentially spoiled for you. It's a risk you face as soon as you click the bookmark or hit enter in the address bar and expecting otherwise purely for the sake of your own convenience is naive and foolish.
If you want to interact with a devoted community then keep up with the subject matter they're devoted to. It's on you to do that, not on everyone else to limit themselves. If it's a really old/established franchise you're just getting into them it's unfortunate but can't be helped. If it's a new show and you're a few episodes behind catch up and then involve yourself.
Respect is mutual, after all. A community should mark spoilers for a time as a matter of respect towards new/slightly out of the loop members but new/slightly out of the loop folk shouldn't take the piss with their expectations and should make the attempt to get (back) in the look as soon as possible.

In principle. That's what I personally think but that's not how I personally act. Does that make me a hypocrite even though it's in favour of a greater good?