we do a lot of behemoth/king behemoth, aspido/admantoise, faf/nidhogg, and other hmn campings. Once their respective spawn windows open up, they have pop times that last anywhere from 30 seconds to 3 minutes, depending on the HNM.
Example: Say the HNM died at 1635 and its respawn window is 21-24 hours later with 3 minute pop windows. Its next windows would be --
1335-1338 -- 21hrs later (window #1)
1405-1408 -- window #2
1435-1438 -- window #3
1505-1508 -- window #4
1535-1508 -- window #5
1605-1608 -- window #6
1635-1638 -- 24hrs later (window #7 - final window)
If you don't have the exact time of death, just wait until a bunch of people camping all start using stoneskin, blink, and other buffs. Gives it away everytime. You won't know the exact time but you will be pretty close.
Remember that when you use the "/clock on" function, the time given is based off your PC to the time servers (like time.windows.com or whatever is use by your PC) speeds. I have seen people have about 60 seconds +/- time from others in the LS. So I would add another minute to each end of the window just incase
