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.