Also, how long does a work need to be out before spoilers are no longer an issue?

"Spoilers for a work are okay after this time has passed" is an okay heuristic in a community where everyone can reasonably be expected to familiarize themselves with the work as soon as possible after it has become available - and nowhere else. You cannot generally expect that simply time having passed from the publication of a work means that people are familiar with its content.

The actual question one wants to ask is "am I communicating with an audience where I can reasonably expect that people are either already familiar with the work, or do not care about this particular detail about this particular work being spoiled". This is a hard question in general, and sometimes "has this work been out long enough for spoilers not to be an issue" works as an adequate substitute question for it, but only sometimes.