I found a transcription of the song and each key change is 8 bars long according to this chart,
along with a melody which he does play on the recordings.https://drive.google.com/file/d/0B5GEPP3zhOrVMDk2YzVmODUtOTE2My00MGE3LTgxNTctZmFiYzE3ZmQ0ODEz/view?authkey=CIvbieoJ