Why silver tarnishes
Tarnish is not dirt and it is not a defect. Silver reacts with sulphur compounds in air, in some foods, and on skin, forming a thin layer of silver sulphide that appears yellow, then brown, then black. Sterling silver is 92.5% pure silver and 7.5% other metals, and it tarnishes somewhat faster than fine silver.
The rate depends enormously on the person. Body chemistry, medication, and even the local air quality make a large difference. Some people wear the same piece for a decade with no tarnish at all.