I think if I've listened to at least enough if an artist's stuff to get a general idea of what they sound like and I've liked just about every song up till that point I'd say I like an artist. There's no black and white threshold for me, and I don't expect anyone else to have heard someone's entire discography to say they like a band either. I may have a higher threshold for how much I've listened to when I say someone is one of my faves though.