I'm a fan of using 7 words or more with a space between each, all lowercase except for things that should be capitalized, like names.
There is maximum length to the passphrases that you can use. At least, that's the case with Trezor. I remember that I couldn't use the full length of a passphrase I initially wanted to use, so I compromised and got rid of several characters. Trezor allows a maximum length of 50 characters when it comes to the passphrases.