Each spawn is 8 hours after you acknowledge the collectibles quest (e.g. you don't even have to get them all or any of them - the timer starts as soon as you click on the quest 'collectibles').
I got mine at 06:30. Therefore my next will appear at 14:30. The last at 22:30. It works down to the second and it doesn't matter if you stay logged in or not. Since I'm a creature of habit and have almost exactly the same life schedule each day, I can tell you that those times are the same for me every day. You can set a clock by it.
In order to "reset" the timer skip a phase. For example be logged out before they appear and don't log in until you want to acknowledge the quest and start the 8 hour timer. Note that having the quest pinned will count as activation, so if you don't want it to automatically start counting when they show up make sure the quest isn't pinned when you ended the last one.
Diwata discovered all of this and I rely on it each day.