Well they aren't in any rainbow tables, so they must be pretty long. Judging by the high reward on this, he probably used 15-20 characters. Enough that you might as well keep your computers mining bitcoins, it could be months even for a very powerful group of computers.
Already at 10 characters alphanumeric, the possible number of combos is 839299365868340224. @5 Ghash/s, it would take over 5 years to go through them all, and each additional character multiplies the time by 62.