The term "prolonged' is irrelevant here.
Major depressive disorder (as well as some other disorders) are classified as single episode or recurrent. An episode is considered recurrent when there is an interval of at least 2 consecutive months between separate episodes. During that interval, the criteria is not met for a major depressive episode.
Criteria for an episode of MDD is at least five of the following lasting at least two weeks: depressed mood, lost of interest or pleasure in most or all activities, insomnia or hypersomnia, significant weight loss or gain or decrease or increase in appetite, psychomotor retardation or agitation, fatigue or low energy, poor concentration, thoughts of worthlessness or guilt, and recurrent thoughts about death or suicidal ideation.
An episode is said to be "in partial remission" when some of the symptoms for the immediately previous major depressive episode are present, but full criteria are not met, or there is a period of less than two months without any significant symptoms of major depression. To be "in full remission", is no significant symptoms for two full months.
Example:
All of 2018, all of 2019, and for 2020, Jan, Feb, March - 6 symptoms present: Major Depressive Disorder, Single Episode, (severity)
April - 4 symptoms present: MDD, Single episode, in partial remission
May - no symptoms present: MDD, Single episode, in partial remission
June - no symptoms present: MDD, Single episode, in partial remission
July - still no symptoms. Now MDD, single episode, in full remission, because two full months have passed (May 1 thru June 30, for example).
August - 7 symptoms. Now MDD, recurrent episode, (severity)