Radioactive decay- half life?

Iodine-131 is used in the diagnosis and treatment of thyroid disease and has a half-life of 8.1 days. If a patient with thyroid disease consumes a sample of Na131I containing 10. �g of 131I, how long will it take for the amount of 131I, how long will it take for the amount of 131I to decrease to 1/10 of the original amount?