Depends on the issues. Some things will change in a week, but they usually did six weeks intervals between my tests until things became normal. But mine wasn't a diet issue.

Too many variables to say. It could be within a few minutes, it could be weeks. It depends what conditions you're looking at, your general health, metabolism, exercise, what kinds of foods.... lots of things.

Depends on what the issue was with the blood work...cholesterol ?

Forever

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.

Until you're no longer obese

Depends on the "crap" in your blood work...