The following is from Vladimir Zak's book Improve Your Chess Results. Zak was a famous trainer in Leningrad, who counted Spassky and Korchnoi among his pupils.
"Annotating a game takes 12-16 hours on average. For convenience, the whole task may be divided into 5 steps:
a) Play through the game quickly, taking 15-20 minutes, so as to call to mind again what you had thought and felt.
b) Go over the game in the course of an hour, and make a synopsis of its characteristic critical stages.
c) In the course of 3-4 hours, analyze the critical stages in detail.
d) Analyze the opening phase, taking care to fill any gaps in your knowledge of this variation. For this 3-4 hours are needed.
e) Go through the game and put together the commentary as a whole (4-5 hours).
Often, in the process of notating, you will convince yourself that your overall plan which did bring you a win in the game was nonetheless faulty. The fact should be self-critically disclosed, and not concealed in the undergrowth of inevitably specious analysis; theory and practice are united. The work finds its culmination in the conclusions about your typical, repeated errors - inadequacy of opening preparation, weak technique in endings, etc."