Re: So - what constitues a suitably strong password
Not all that long. 9 random characters is probably fine.
If you've got $60 worth of bitcoins then using a random 9 digit number (which obviously has 10e9 possibilities) as your password would make it not currently worthwhile cracking your password. Guard against a 10x increase in performance / reduction in cost by using a 10 digit number. Alternatively, use a random password of upper case, lower case and numbers (62 possible chars). That would only need to be 1/(log(62) times as long which means 5.58 random characters will do it.
6 gives 56 x 10e9 possiblities.
9 gives 13,537,086 x 10e9 so should give plenty of room for the possiblity that you're storing more than $60 worth and increases in processing power for the near future (unless quantum computing hits the big time soon in which case this is all moot).
Final option, use the xkcd method. Pick 4 random words, even if you only pick from the 1000 most common that's 1000^4 = 10e12 possibilities