Weed seeds can survive in the soil for years before they germinate and grow. Weed seeds in deeply worked soil tend to last longer than seeds in shallowly worked soil. Grass seeds tend to be less persistent than broadleaf weed seeds. The number of surviving seeds of most weed species declines rapidly the first year. But thereafter the rate of weed seed decline slows.
Some seeds can persist for decades.
Seeds excavated from a monastery in Denmark were dated to be 600 years old and 11 of them germinated. More commonly, mustard seeds last for decades.