Summary: Linux 6.19 adds a new listns(2) system call that makes much easier to list the namespaces present on the system; support for the Live Update Orchestrator, which allows to reboot a kernel via kexec while enough kernel state to allow virtual virtual machines continue working after a reboot; support for PCIe Link Encryption which lets PCIe devices encrypt its communication with confidential VMs; Btrfs support for the experimental shutdown ioctl and suspension during scrub or device replaces; Ext4 support for block devices larger than page size and faster online defragmentation; support for the color pipeline API for better and faster HDR graphics; improvements to io_uring; and support for the SFrame format that brings faster frame unwinding. As always, there are many other features, new drivers, improvements and fixes.
(Summary copied from the changelog at kernelnewbies.org/Linux_6.19)
Depends entirely on your distro. Some distros, within weeks, other distros will take up to a few years. Just depends on whether your distro prioritizes bleeding edge or stability.