When does a spoiler in a video game start to be o.k. to talk about?  

Some spoilers, like Sheik's secret Identity in Ocarina of TIme, or the final boss of Twilight Princess, or Liquid Snake's parentage, are common knowledge, and not taboo to talk about.   However, I believe everyone's enjoyment of the game would increase if it was a suprise.  When does it become O.K. to openly discuss different spoilers?  is it a set of time?  or the way the developers handle it?  I'm curious while some spoilers( i.e. Rockstar game endings, The Walking Dead, Journey) are still handled sacredly even though a bunch of time has passed since their release as well.

If a game is highly memorable and treated like a classic, then it can't be counted as a spoiler.

For example, you can't get mad if we tell you that bowser dies in Mario. You can't angry if you are told that Darth Vader is Luke's father.

On the other hand, if I told you the end to, say, a professor layton game, I'd be an asshole. Even though the series is popular and successful, it isn't as recognizable as a mario game or star wars.

Only exception to this is for games with obvious endings. No point in throwing a tantrum because someone says you end up saving the day in Kirby. It's common sense.

I always figure big spoilers should never be treated as non spoilers until a sequel comes out and through its very presence spoils those spoilers.

Of the three spoilers you posted (didn't actually spoil any of them, thank you). I know two of them due to having played the games.
I have no clue who snakes parents are. Never played MGS. Not sure what would be common knowledge about it. I'm not sitting at the water cooler and someone starts talking about snakes daddy.

Heck, even movies people should shy from mentioning Luke's parents, or the sixth sense, ect.

Math.Pow(2, Metacritic score) = number of days before spoilers are ok

A perfect 10, almost 3 years
A solid 9, about one and a half years
A great 8, close to 9 months
A passable 7, 4 months
A meager 6, 2 months
A crappy 5, 1 month

Math solves all questions!

I mean, I don't know any of these things.

I feel like at least a year cutoff should be the minimum.

The more popular, the quicker it becomes acceptable to talk plot details. Look at major movie releases - spoilers are frowned upon the first two weeks, but after that its funny and acceptable to make and share spoilery memes on social media. The timing for games is quite different, but still there.

I never fought the final boss in TP because I got bored to death by the second Wolf Link section and dropped the game. So I still consider it to be a spoiler and do not want it to be revealed. There are always many people that never played the game (like Playstation owners) and they should have a chance to enjoy it.

An absolute no-go was the reveal of a major spoiler in the trailers for Xenoblade 3DS, it would have ruined the game for me.

I feel like smash bros spoiledparts of xenoblade for me

I use the 1 year rule. I consider any spoilers defunct 1 year after release. If you haven't watched, read or played it by then you obviously didn't care very much.

Same. Its simply not reasonable that I can't talk about Xenoblade Chronicles freely. Who is honestly going to buy a wii, or 3ds and get that game now?