Moving $CKB and other assets from Layer 2 to Layer 1 is done using a special transaction that is picked up by the L2 validators to release funds back to L1. This withdrawal process takes approximately 3 days to complete.
The reason that the withdrawal process takes 3 days is that the L2 validators need a long period of time to be able to challenge fraudulent requests and ensure network security. Security is prioritized over convenience on core pieces of infrastructure. This 3-day window may seem excessive, but this window is sometimes twice as long on other popular blockchains.
In the future, faster withdrawal will likely be possible using alternative methods through third-party service providers. These services will offer faster and more convenient withdrawal services, but may not have the same level of security or decentralization as the official 3-day process.
At present time, the withdrawal transaction must be generated using a dapp. Users of the popular YokaiSwap will find that everything they need to move $CKB and other assets between L1 and L2 is available on the YokaiSwap website. This is the recommended way to handle this action since wallet support for L2 is still being developed.