7-Zip uses iterated SHA-256 as its key derivation function. This is weak against hardware brute force attacks. If your password really is 18 randomish characters, you should be fine. If it's one English word with a few digits before or after it, you are theoretically vulnerable to that kind of attack.

On the bright side, you don't really have to worry about someone stealing your wallet today and then breaking it in ten years when the computing power is available to do so. Shortly before the time any encryption scheme you ever used to protect your wallet becomes vulnerable to an attack (due to increasing computing power, a newly-discovered flaw, or whatever), you can simply transfer all your BitCoins to a brand new wallet using an encryption scheme that is stronger.