It all depends on the amount put in you may not have to drain your tank at all. If you put in �10-�20 worth then just fill the tank to the top with diesel. When you have used a quarter of the tank,fill it to the top with diesel again. Repeat this about 6-8 times. Your car will be fine,how do i know? I did it myself last year. Though if you`ve put more than �20,more than half a tank of petrol in your car then i would suggest getting it drained by a professional mechanic or garage.