Extracts of St. John's Wort are standardized by their concentration of hypericin even though it is not completely clear that it is the hypericin that gives the herb its anti-depressive properties. In fact, it is the hyperforin substance that has been found to be a serotonin-reuptake inhibitor, the very same property that gives drugs like Prozac and Zoloft their activity against depression. The recommended dose of St. John's Wort is 300 milligrams taken three times daily. The only significant side effect is that of hypersensitivity to the sun if taken in large doses. I think the key point to remember is that depression can be a serious thing - far more serious than just a few days of the blues.
Serious depression needs medical attention and probably conventional medical treatment.
Don't diagnose or treat yourself, and try to see a provider who wouldn't be opposed to letting you try St. John's Wort if it appears things aren't too dire. Used under License.