In most cases bounced emails appear within minutes of delivery. However, some may take up to 4 days to appear as bounces. You can monitor this by visiting the "Bounces Over Time Report" report frequently. You will also notice your email campaign statistics change during the 4-day period after sending a campaign.
A bounce is a message that comes back from the recipient's mail server, saying the message could not be delivered. That can be a permanent failure (a hard bounce) or a temporary failure (a soft bounce). We will only show a bounce in your reports once all the attempts to redeliver a temporarily bounced email have failed.
For that reason, it may not show up for some time.