The mail forwarding settings in cPanel will reflect in real-time, meaning any emails sent will forward immediately.
Note: However, it may sometimes be delayed, depending on the congestion status of the mail forwarding on our server or the mail server at the forwarding destination.
It may also be affected by the congestion status of mail, such as the route from the sender mail server, the route to our server, and the route from our server to the destination mail server.