Years? IDK, but you only need to add one line to your configuration.nix to switch between them. AKAIK #NixOS defaults to #Xorg on install.
But #Wayland is the future.
Discussion
Yeah, probably you'll have many years with #Xorg on #Linux, that includes #NixOS. Because it isn't dependant on which distribution you use, but on which #DesktopEnvironment. i3 have already announced they'll never switch to #Wayland because of the Sway fork (same keybindings as i3 but using Wayland instead of Xorg).
And many smaller projects (LXQt and Xfce come to mind) will take years to develop a version that works on Wayland ✌️😎