You must be thinking that such a big monument needs cleaning very often. No, you are wrong.
Mount Rushmore did not require too much of cleaning. The only thing which was required to be done very often was to maintaining it to keep the monument intact.
Are you thinking that why was this monument not cleaned often? Let me help you with that. The main reason of not cleaning it often was the budget.
This went better when a company named Kärcher took the responsibility of cleaning the monument and did huge favor to the country in the year 2005. This company would use their own equipment and the cleaning of the monument would take weeks.