The duration varies, depending on if you transfer from one package to another package or from one account to another account.
Transfer from one package to another package in same account
It takes 24-48 hours for the DNS to fully update and the changes to appear live. However, it is not uncommon to see the domain transfer appear complete in the 1&1 Control Panel in less than 12 hours.
Transfer from one account to another account
The domain transfer will have to be approved by the current holder of the domain in their 1&1 Control Panel. After the transfer has been approved, it will be 24-48 hours until the transfer is complete.
Before transferring a domain, backup all website files, databases and E-mail accounts. Take note of any DNS settings or subdomains created under the domain as no data or configuration will transfer with the domain.
For additional information, you may want to reference: