Hi, medical scientist here. HA1C (average reading of blood sugar)takes about 3 months to completely update. But that a rolling average so it starts to change day by day in small increments.
Glucose changes by the minute based on food consumption and activity so it's only a snapshot of that particular moment.But it's still very important because when fasting you should remain at certain levels if your body is processing sugars correctly. Cholesterol takes about 3 to 6 months to effect change using diet and exercise alone. Without knowing further details I would suggest 3 months.