"Passano sette lunghi anni."
Can this also mean ' They spend seven long years', or would this use a different verb?
Yes and no. It could mean that, but then you should say WHAT they do in those seven long years.
"Passano sette lunghi anni a ricercare, senza trovare la risposta."
Does it make any sense in English to say to spend time without saying what for? (I ask, I don't know!) :)
You're right. Unless a question had been asked making the acitivity clear, the sentence would say what they spent the seven years doing.