The day cycle is a period of 20 hours in game from 6am to 2am. Past midnight (12:00 am), the player's character becomes tired and in need of sleep. However, the player can stay awake until 2am, after which their character will pass out.
Stardew Valley is a pretty lengthy game. As you play on your farm, you will come across various milestones and progress trackers, but there is no end to the game. Players can continue farming for years in the game without growing bored.