What are the general timing guidelines for an employee drug test?

Q: “Our business is in California. I read that if an employee tests positive for drugs, he/she has the right to get tested in any clinic of their choice within 10 days of getting the positive test; also, that they can be tested up to 72 hours of being asked. Can our employer make it mandatory that they get tested right away, and that they do not have the option to get a second exam?” – Quintila, California