3 ms·
http://xkcd.com/936 http://xkcd.com/936
http://xkcd.com/936 http://xkcd.com/936
A random 12-digit alphanumeric is log((26+26+10)^12)/log(2) = 71.45 bits of entropy. Plenty for a web form.
You use a strong passphrase to secure your password database. You allow your password database to generate 16 or whatever character length passwords.