I recently killed a leader and got a bad omen effect which doesn't have a time limit, it lasts forever. Also even if I drink milk, so that the effect disappears, when I leave my world and then reopen it, this effect is still there with no time limit.
This feature works as intended, Bad Omen effect is a increment value that decreases, it can last as long as five minutes or more, and will wear off only if you die or encounter a village, of which a raid will begin.