This is "simple" in its most basic form:
1) How long does the information need to be protected?
2) At what rate can the attacker guess passwords?
But of course defining those things is hard. Only you can say #1, but your answer is probably not a thousand billion years, so that gives you somewhere to start.
As for #2, attackers have wildly varying amounts of horsepower at their disposal. Like @Zoredache said, you have to decide what level of attacker you're worried about.
If you're worried about your roommate cracking it, and the information only needs to be protected for a couple of years (i.e., it's of no use after that time anyway) then just pick a password that's as strong as any other place would recommend (see the XKCDs mentioned above for a good summary).
If you're worried about the FBI cracking it, and want it to be protected until you're dead, you're in a whole different ballgame. These kinds of "attackers" will have dedicated, special-purpose hardware at their disposal that can rip through hundreds of thousands or millions of passwords per second. And that's today. Ten years from now... you get the point.
But, since we're talking filesystem encryption, this is all to presume that the attacker has gotten ahold of your drive, knows what software and algorithm has been used to encrypt it, and cares enough to do so. So if you keep your system in a safe environment and use something such as TrueCrypt's hidden volume feature, you've already greatly mitigated the risk. Pick a strong algorithm, such as AES256 as you've mentioned, change the key once every year or two, pick a reasonable passphrase and, unless you're doing something highly illegal or sensitive, I wouldn't lose any sleep over it.