I attack anything that I am able to attack.
I will spend the required 24hrs in my square and then explore the adjoining squares until I run out of torpedoes, or fuel, or am damaged enough to require going home.
On my present patrol I am returning to base after about six weeks on patrol with 5 torpedoes left and 2 merchant ships sunk. It should take me a week or so to make St Nazaire so I might still run into something.