Cobalt 60 is a radioactive source with a half-life of about 5 years... ?

15 yrs if you were to set it at 5 year half life (I think it actually is 5.2 years). 5 years would put it at 50%, 10 years would be at 25%, and finally 15 years would be 12.5% or 1/8 of original strength regardless of the beginning activity.