In a study of children in schools, setting up a 'handwashing event' 4x per day cut sick days in half. Another controlled study on hand sanitizer showed a 20% decrease in sick days.

Here's a replication on sanitizer showing 20% reduction in illness rate for college students.
http://www.fda.gov/ohrms/dockets/dailys/03/Sept03/090203/75n-0183h-c000075-att-14-vol154.pdf

Here's one in Pakistan where incidence of Pneumonia and Diarrhea in children were cut in half.
http://www.ncbi.nlm.nih.gov/pubmed/16023513

Here's one in China where intensive handwashing programs for child schools cut sickness by ~40%. http://www.ncbi.nlm.nih.gov/pubmed/17556631

The short of it is 20% to 50% fewer contagious sicknesses, depending on your current habits (assuming what holds true for children or college students holds true for you).