I get 24592 3-7 letter words, which a reasonable set to use for a passphrase:
$ grep -E '^[a-z]{3,7}$' /usr/share/dict/american-english | wc -l
24592
That gives 15.076 bits per symbol as opposed to 5.17 for single-case alphanumeric (my usual choice for memorability and efficient entry). That means passwords of the form of correcthorsebatterystaple are in between an 11 character and a 12 character alphanumeric password in strength, assuming you use a good RNG for generating passwords and your attacker has full knowledge of your dictionary. To get a random english password of equivalent strength to a 20 character alphanumeric (my standard for encrypted disks), you need to use 7 words. That might be reasonable if you are a relatively fast typist with a relatively poor memory.
If you wanted your random english password to be as strong as the AES key you're deriving from it, you'd need 9 words.