Cobalt 60 has a half life of 5.27 years. Which means, at less than the date of 5.27 years later, the Co60 would be at the same radioactive levels as many common substances we come into contact with everyday.
That is overly simplified IMO. That all depends on the quantity of CO-60.
If you had 1 gram, then after 5.27 yrs, 1/2 of that gram would be CO-60, the other 1/2 it's decay product.
then in another 5.27 years, 1/2 of that 1/2 gram of remaining CO-60 would be it's decay product, leaving you with 1/4 of the original amount. and so on and so on.
If the quantity was small, the total radiation dose after 1 half-life could indeed be at or below the background radiation level, however, if you had a large quantity, it would still be deadly radioactive, especially in the case of CO60 due to it's gamma ray emissions.
No, I do not work in the nuclear industry, but I was trained a few years ago, as a member of a volunteer urban SAR team, on radiological detection and monitoring....
Sol