There is no benchmark for this. This might take several minutes if the cloning involves creating a virtual disk. You can cancel the cloning at any point before the customization stage.
As was mentioned, the longest part of the cloning process is copying the virtual disk. Even if the virtual disk is thin provisioned, there's likely to be at least the OS and binaries that need to be copied to the new virtual disk. Your underlying storage infrastructure performance has a lot to do with how long the process takes.