Eventually every new NixOS user is going to run out of disk space. Or, atleast I did.
Here’s how I did my housekeeping:
# Keep 5 system generations
sudo nix-env --profile /nix/var/nix/profiles/system --delete-generations +5
# Same for your user profile
nix-env --delete-generations old
# Run garbage collection (this actually frees space)
sudo nix-collect-garbage -d
# Clean boot entries (removes old kernels from /boot)
sudo /run/current-system/bin/switch-to-configuration boot