How long does minor depression last of not treated?

Depends: It can last from days, to weeks to months or longer. Minor today, major tomorrow. It would depend on the circumstances and the person and the way the person responds to depression and the events leading up to this. Remove the cause and the depression often goes away. Not always though. I would recommend talking to someone. Perhaps you should start with your pcp?

Just a definition: Major depressive disorder is a checklist of criteria for a specified duration. The condition fulfilling some of these criteria may go on to fulfil the criteria for major depression if untreated, or remit on its own.