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.
An old trick used by us old trucker during the winter we would deliberately add petrol to ouor diesel tanks in order to prevent the diesel from freezing. Therefore, simply add as much diesel as possible to your tank and carry on driving as normal.