A major reason that the Holocaust is taken very seriously is that there are people who believe that doing so will make a repetition less likely. I don't know how long it would take for that to fade out.

That the Jews were slaves in Egypt [1] has been commemorated every year for at least 2500 years-- possibly 3000 years or so.

I wouldn't expect it to fade quickly.

[1] This is disputed-- there doesn't seem to be any solid evidence of it.