How can I know the duration or the term of a task?
I'm not sure what you are asking.
If you are asking how to know how much processing time each task gets then you can use the run-time stats feature. Google "FreeRTOS runtime stats".
If you are asking to know the time between when a task starts running and when the task stops running each time the task runs, then you can implement a traceTASKSWITCHEDIN macro. Google "FreeRTOS trace macros".
If you are wanting to profile your application then you could use FreeRTOS+Trace http://www.freertos.org/trace.