I have a brainwallet with a passphrase of two sentences that I'm almost certainly never going to forget. It has unconventional punctuation, made up words, and it doesn't appear anywhere either online or in print. Is it safe to assume that this will be safe for a long, long time?
I don't think anybody knows. But you can find out:
Send a token amount of bitcoin to two more brainwallets, made from each of the sentences.
When somebody else either spends those bitcoins or sends more bitcoins to them, you know that your main brainwallet isn't safe any more. (are the services that will send you an email when there is activity on a bitcoin address still around?)