They are fixed, every 6 hours. However, the only one that is different is the first time you log into the game starts the clock on all your shipments. So if you got a shipment at 6PM and then do not log in at all until the next morning, that's when your next Shipment timer starts.
I think someone mentioned something about keeping the game open to maintain a Shipment time you like or something, but that seems a bit complicated. I'm usually happy if I can get snag 2-3 shipments purchases a day.
Easy way around this: suppose you sleep from 11 pm to 7 am. Log in (or refresh shipments) at 3 or 9 pm or am. Get the shipments just after 9 pm, then leave the app logged in. That 6 hours will expire at 3 am and a new one will start, so you can collect again after you wake up before 9 am.
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I think someone mentioned something about keeping the game open to maintain a Shipment time you like or something, but that seems a bit complicated. I'm usually happy if I can get snag 2-3 shipments purchases a day.
So I guess I should rephrase my post. I wish the developers would just have fixed shipment times.
Cosign.
They are fixed. Every 6 hours they will refresh. However if you are not online at the refresh then it won't count down until your next login time