Modern phones can store up to 256 GigaBytes of data on them. Transferring anything more than a few GB of data can take several hours. In most cases, if the transfer status shown on the screen indicates that the transfer is still progressing, then it’s just a matter of waiting for the data transfer to complete.
To minimize the time taken by content transfer, encourage your customers to activate their iCloud or Google account and sync their data to their account. This will enable them to sync their content to their new device by simply activating the account on it.