How many 'random' characters should my iCloud password be if it were going up against the BEST hackers and the best hacking software?
I think this question can't be answered by just typing a number. There are things to consider:
- How are the passwords stored in the database? E.g. what hashing algorithm or encryption is used.
- Does iCloud use salt? If so, is this salt static for each user or random?
- How well are the support people trained in order to detect a social engineering attack? (as we've seen before at Apple: http://www.wired.com/2012/08/apple-amazon-mat-honan-hacking/all/)
- How much data is available on social network sites about you? This because of the questions asked in order to get access to an account: http://www.washingtonpost.com/news/the-intersect/wp/2014/09/03/this-is-how-easy-it-is-to-hack-someones-icloud-with-their-security-questions/.
Having said that, I believe the iCould (I am not a user there) can be configured with two factor authentication (something you have and something you know) which you should consider enabling.
Having two factor authentication enabled and a password of 15+ characters can be considered safe. However it's also a matter of how much effort one wants to put in to hacking your account (e.g. whether you're a celebrity or not)
Hope this helps, as you can see there's more to it than just a number of random characters.