The radioactive half life of the seeds (iodine-125 is 60 days and palladium-103 for 17 days).
So after 4 half lives (8 months for iodine and 2 months for palladium) the seeds should only have 1/16 or 6% of their radioactivity left (1/2 X 1/2 X 1/2 X 1/2).
Scintigraphy is another way to follow up for migration of seeds.