For symmetric keys, since they cannot be weakened using quantum computing, their strength can be assessed by their bit-equivalent amount of entropy:

• 40 bit or less - easily breakable
• 64 bit - not so easy, but doable
• 128 bit or more - basically unbreakable

Those are equivalent to, respectively:

• 0-9 - 12, 19, 38 characters
• a-z - 9, 14, 28 characters
• a-z0-9 - 8, 12, 25 characters
• A-Za-z0-9 - 7, 11, 22 characters
• A-Za-z0-9+special - 7, 10, 21 characters

Moral of the story: drop the special characters, and even the numbers... and even the uppercase. A 30+ character long all-lowercase pass phrase, is already unbreakable.

Check @falsemirror@beehaw.org:

finance-caffeine-utopia-redress -unseen

...is already over 128 bits.

PS: Correct horse battery staple