As you mentioned in your own post, it depends on their lifespan.
Furyosa stays for 4h (and her demons too), so she won't spawn after 6 AM CET (4h before ss).
Dharalion lives for 8h, so it won't spawn after 2 AM CET (8h before ss).
Most common bosses live for 1h, so if it's less than 1h before ss and someone already checked them, not worth checking anymore.
Banks event can happen every hour starting from server startup (not login time, but about 3-4 minutes earlier). You can track server startup time when Kingsday event is active, as hourly raids happen at the same time. It's usually from 10:02:45 to 10:03:30 CET.
Oodok Witchmaster and Arthom the Hunter can spawn anytime and will stay forever until ss or until killed.
Variable spawn nemesis bosses, like Hive Outpost and Mahatheb, or the otherworld minis, will also stay until SS if their spawn is forced.
And even tho this is not a nemesis boss, Albino Dragons live for 30 minutes, and can spawn every 30 minutes starting from 10 minutes after server startup.