St. John's Wort has been demonstrated to be effective for the treatment of depression when given at doses of 600, 900, or 1200mg/ daily most commonly given in divided doses. Most studies evaluating the effectiveness of St. John's wort were at least 4 weeks in duration and in these studies the drug was given routinely each day. To receive the desired benefits of this medication, you should generally be taking it on a daily basis (not missing days of use) either as short-term therapy, or for long term treatment, but with a limit of one year (as reported from previous studies).
To avoid the interaction with tramadol, you may consider an alternative pain reliever which does not possess the characteristic of increasing serotonin. There are numerous products to choose from, and your doctor can help to determine which one is right for you. If you decide to take these medications together, you should use them with caution, and I would recommend you start with the lowest effective dose of St. John's wort, such as 450mg/ day and increase to 900mg/ day if necessary.
You should make sure not to take any additional medications that may result in serotonin syndrome, and continue checking precautions of both prescription and over the counter products. The serotonin syndrome is typically characterized by at least 3 of these symptoms: agitation, diaphoresis, diarrhea, fever, hyper-reflexia, incoordination, myoclonus, shivering, or tremor. Monitor closely for any of these changes if you begin taking St. John's wort.