The expiration time is different for each message.

1. Messages that are marked with 'Time Left'
You need to collect the item indicated in the message within the time limit set forth. If you don't collect the item in time, the item will disappear.

2. Messages that aren't marked with 'Time Left'
This is a message that includes items or messages that do not expire. It won't disappear from your mailbox unless you collect it.