Works best when your life is already out of your head. The more you've written down, Allen says, the more the review is worth.
Works best when you match the rhythm to how fast things move. At 43 and at 60, Allen sometimes needed something like a weekly review two or three times a day, to lift his head and ask what the afternoon was for. At 80, with a simpler life, he looks when he feels the need.
Fails when there's no system under it. With nothing captured, the review has nothing to walk. Even so, Allen says an hour a week helps anyone. Spend it asking what happened and what to do about it, on the work of your life, not zoning out.
Fails when it stops. Allen calls it probably one of the hardest habits to set up. Skip it and the system goes out of date, and you fall off the wagon fast. Planning the week is a separate job. Cal Newport's weekly plan does it, and his time blocking turns it into a schedule for each day.