Charitably, it might be viewed as a minor spoiler in that it implies that the character is alive in that book, which is not the first one of the series. (Although that is not a necessary implication: he could possibly be saying it in someone else's flashback, for example.)

Have you seen The Passion yet?

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.

Macbeth dies at the end

Damn you.

this comment on the recent Reddit thread about intellectual jokes goes one better (and actually made me laugh out loud the first time I read it).