It doesn't matter when you collect the last one, it's when the quest becomes active. I do this every day like a ritual (OCD to the max) so my day goes like this:
06:00 - 06:10.... log in, check the quest list for Find all Collectibles. Click on it and pin it so that it shows on the screen. Now I know the 'timer' has been set for 8 hours, I will see the next drop 8 hours later.
Find all the collectibles, doesn't matter how long it takes as long as it's before 14:00
14:00 - 14:10. Be logged in, wait for collectibles. Since the server resets at that same time in NZ I also refresh just to be safe because sometimes the collectibles show up but I don't get a notification unless I refresh. When I see the quest in the list and have clicked on it I know the timer is set for 8 hours again.
Find all the collectibles.
22:00 - 22:10. Repeat as above. Activate the quest. That's the key bit to controlling the timing. Then I look forward to another morning
If I miss a window, I often choose to log out before the night spawn happens and I don't log in until the morning. I can then make the 8 hours start at whatever time will suit me for the next day.