[Matrixbeast]

A lot of these shows weren't even available to English speaking viewers until the past few years, though.
There isn't a set time table or grace period for when something should be marked as a spoiler, it all boils down to the poster's call.

You just have to make a judgment call, but also keep in mind certain facts.
For example, Ohranger was available to others, subbed for, about 9 months, and now it's completely unavailable until it's release by Shout Factory later this year.
Using the prime counter argument, 9 months was plenty of time to watch the series in full, so theoretically it's fully justified to post every little event that goes on in it because the series aired 20 years ago, and the subs were available for 3/4 of a year.
But is it fair to the people who didn't download it before all the subs were swiftly taken down, and now have to rely on streaming sites of lower quality and dubious nature?

One has to post spoilers at their own discretion because there is no golden rule.