Why Do I Experience Recurring Depression Every One To Two Years, And How Long Should I Take Antidepressants To Manage It?

I m suffering from depression very long time. i m getting depression every one year once im getting mania also but manic episode is very less compared to depression why i am getting depressed every two years once from 18 years onwards up to knowledege what is casue of continous depression in my life what is treatement for depression how many years i have to take antidepressants to stop depression