It takes me some time to learn a tune but it would be a very long time if I didn't write down music. I make notes, write down riffs, repetitions and a general flow of a song. It's not a very precise notation, but it serves me well. Once I have the song down, it's not a problem to play it at a rehearsal. Have a look at two songs I will transcribe soon in a more proper format, Necros's Search for Fame and Bad Posture's Get Tough.