Do you need to increase your serotonin levels every few years or will the antidepressant you originally were placed on keep working?

Anti-depressants are: a mixed bag... I've known people who have been on the same one for 10+ years with consistently good results. I've known patients who get a good result, yet in a matter of months, the effect fades. We don't know why, but when the effect thins, most psychiatrists try another medication, usually in a different class. Some will augment your existing med first with a second medication.