Anything between negative infinite and infinite.
Bosses which are part of raids (announced or not) and floor resets are completely unrelated, that source you quote is simply wrong. If that was correct then it wouldn't be possible to have raids happening right in front of you, which happens every very often.
Floor resets only happen a few times a day on most places and yet raids can begin at almost any time and cannot be blocked by players, otherwise we wouldn't have seen Ferumbras, Orshabaal and the likes ever spawning anymore since there are always players there waiting for them.