Ideal passphrase length: old diceware method (5 words) vs. your Bitcoin wallet.dat passphrase length (8 words) and doubling passwords?
It is all a matter of how many bits of entropy your passphrase has (how hard it would be to guess). Using a couple words provides a lot more randomness than a couple random characters.

As far as I remember, the algorithm used to encrypt the private keys that is used by BitcoinQT has about 100 bits of security (which is a lot). You would want to attain a similar amount of security with your passphrase.

The english language has about 171000 words in it. But lets face it, most people won't use more than 60000. Assuming that your phrase would consist of lets say only the 10000 most popular words, you would need log_10000(2^100) words to achieve 100 bits of security, or about 7.5.

So using 8 words should be enough to secure your coins for a couple decades at least (assuming the current rate of computational speed growth). However, once the quantum computers enter the picture and become powerful enough, all guesses made now can be tossed out.