Overview: There's actually a series of bugs while using the /effect command in singleplayer (cheats-enabled) mode. According to Minecraft Wiki, the duration is counted in seconds and shall not exceed 1,000,000, but in practice the status effects will actually last for an unexpected length of time. It seems that for different effects, the timing is treated differently as well.

Problems:

• Using effects No. 1-22:
  1. The number of seconds is consistent and correct ONLY if it lies between 0 and 1638.
     
  2. If the duration is set to at least 1639, the timer freezes (＊＊:＊＊), meaning the effects will last forever until it's cancelled with milk or commands.
     
• Using effect No. 23 (Saturation):
  1. The number of seconds is inconsistent with your command for ANY durations set.
     e.g. Saturation for 1639 seconds (=27m19s) returns as around 1m21s
     
     e.g. Saturation for 100 seconds (=1m40s) returns as around 4 seconds
     
     e.g. Saturation for 20000 seconds (=5h33m20s) returns as around 16m38s
     
  2. If the duration is set to at least 32767, the timer freezes as well.
     e.g. Saturation for 32766 seconds (=9h6m6s) returns as around 27m17s
     
     e.g. Saturation for 32767 seconds (=9h6m7s) returns as infinite duration.