Periods is a plural noun for a word "PERIOD."
Period: 1. A length or portion of the time.
2. A portion of time in the life of a nation, civilization, etc. characterized by the same prevalent features or conditions.
3. A major division of geological time that is a subdivision of an era and is itself subdivided into epochs.
It was in the way Manpreet stood, gazing absently out the window for long periods of time.