https://en.wikipedia.org/wiki/Zinc_and_the_common_coldtrials indicates an average 42% reduction in the duration of colds
The duration of nasal discharge was shortened by 34%, nasal congestion by 37%, sneezing by 22%, scratchy throat by 33%, sore throat by 18%, hoarseness by 43%, and cough by 46%. Zinc lozenges shortened the duration of muscle ache by 54%, but there was no significant effect on the duration of headache and fever