How much time do you have to be clean of marijuana in order to be a police officer?

It will depend on department policy, but some departments may not consider you if you have ever smoked marijuana.

Consider that having smoked marijuana, you have broken several state and Federal Laws.

Here in Anchorage,Alaska it is 3 years.I know because I looked up department regulation,however it may be different in other States with stricter Marijuana laws.