Maximum Task Duration (In hours)

There is no Scrum rule regarding the duration of a task.  In fact tasks are optional.  A recommended practice is 8 hours or less for tasks in the Sprint Backlog for many teams, but the Development Team should come up with that norm.

Hi Fabio,

There is no such rule in Scrum where a task duration under an user story cannot be estimated longer than 8 units.

But as Scrum is based on Empiricism, it is recommended to break larger Stories into smaller ones of 8 and less Story points. 

Thanks.

Task should not take more than a day or two to get completed thus there is a relation between task estimation and working hours. Let me know your thoughts on this.

Fabio, note in Scrum there is a key inspect & adapt moment which is called the Daily Scrum. Tasks are covered with the ultimate aim of completing stories. Stories could be linked to a Sprint Goal. See where I'm going?

But as Scrum is based on Empiricism, it is recommended to break larger Stories into smaller ones of 8 and less Story points. 

Story points are also optional in Scrum. Even if they are used, there is no unified definition of a Story Point, so for some teams, 8 will be small enough; for others 8 will be far too large. This is also subject to empiricism.

I encourage my teams to create tasks that they feel can be done in 1 day or less.  It provides them a way of seeing progress each day at the Daily Scrum and gives them a feeling of accomplishment.  If they do create a task that is going to carry over multiple days based on information they find while doing it, I suggest that they break it into smaller tasks, put them on the board and remove the larger task.  This allows them to keep track of what they have done, the team can better understand what they are doing, and it is possible that another team member can take some of the work to get it done faster.  

There is a lot of psychological sub-tones to agile practices that are not always made apparent to the people doing the work.  But as an Agile Coach or Scrum Master it helps to understand them so that you can actually use them to the team's advantage.