Hi , I am not sure about how much time it takes to restore serotonin, but I could say what you could do to restore it instead of depending on tablets. Though you take tablets , you start with one small pleasurable activity that you are interested. Like watching sunrise , sipping a favourite drink Etc. Observe how you enjoy it. Start realizing that you can bring happiness by doing something which you enjoy. This will stimulate serotonin. Because as you focus to enjoy small thing , your thoughts that moment will be positive only. Positive thoughts raises your serotonin. So you can indirectly work on your brain chemicals and also bring change in your behaviour slowly.