Depends. The effects of a medication like Prozac (fluoxetine) depend on why you are taking it. In cases of depression, a positive effect can be seen in as little as 1-2 weeks. However, for anxiety disorders, especially hard to treat ones like ocd, effects sometimes take up to 2 months to be seen.
About 6 weeks. Many antidepressants require 4 to 6 weeks to build up to a therapeutic level in the body.