Lifetime of unread push notification

Hello guys this is simple question for anyone that already know this.

I just want to know, once i send a push notification to an IOS device, how long does it lasts in his screen while the IOS device is in sleep mode? It is important for the design o开发者_如何学运维f my app.

If they dissapear even if they have not been read, then i'll have to push at a different time.

Thanks so much!

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20 seconds, if you have not received another notification.