Mostly self contained within the spreadsheet.
Approximately 2h after the last kill (give or take 1min), a message will be broadcast on the map announcing that it will spawn. Approximately 9 to 11mins after that, the boss will spawn. Therefore, the total respawn time is 2h 9min to 2h 11 min.
Spawn times starting from a server maintenance can be inconsistent, and bosses may very spawn much later than anticipated.
Channel crashes (usually Klaipeda ch 1) can also affect spawn times, pushing them around by up to an hour.
If you’re looking for Demon Lord information, see above thread.