How soon after eating food high in cholestro will the test result be influenced?

Cholesterol tests are recommended on an empty stomach.  Plan on a good night's sleep and an early morning blood test, before breakfast, for the best results. 

Your cholesterol level should always be measured after an overnight fast -- if you eat before your test, the measured level will be much higher. 

Perhaps you were asking how long does it take for your fasting cholesterol level to fall after you lower the total cholesterol in your diet -- if you significantly restrict not just your cholesterol, but your total fat intake, your fasting cholesterol should fall within a couple of weeks.  For good measure, start a routine of regular aerobic exercise, and make sure you have lots of fiber and green leafy vegetables in your diet!