Why Silver Tarnishes
Tarnish isn't dirt, and it isn't a defect. Silver reacts with sulfur in the air, in certain foods, and in your skin, forming a thin layer of silver sulfide — first yellow, then brown, then black. 925 silver is 92.5% pure silver and 7.5% other metals, so it tarnishes slightly faster than fine silver.
How fast varies enormously from person to person. Body chemistry, medications, even local air quality all play a role. Some people wear the same piece for ten years without it darkening.