How many times should one wash impurity off one's hands?

I want to ask that if one's hands are very dirty e.g. if urine drops can be seen on one's hands then how many times should the hands be washed. Beyond that limit of washing, will it be a sin. I'm asking this because I'm getting mad day by day as I wash my hands for 20 to 25 times with soap, so it very irritatating to wash so many times as I'm not getting satisfied that my hands are now clean.