How much should the scrum teams commit for in sprint

Sprints are driven by commitments that a scrum team provides for the stories that it will deliver by end of sprint. This is where most of discussion is about about how efficient a scrum team is in delivery. Although I beg to differ with this practice of linking efficiency and stories committed for - I always do empower my teams to set minimal expectations and deliver maximum. I will blog on this topic in future, but for now how much time is available to plan for?

Following are the main categories of time in a sprint from planning perspective:
1. Vacation / holidays
2. Time for scrum mandatory formalities
3. Time for so called the over head activities
4. Time for unplanned
5. Net planable time

Vacation / holidays : The time when team members aren't available for work due to vacation / leaves / holidays. Some teams do maintain a calendar of leave plans and place it near standup corner.

Time for scrum mandatory formalities : The time for planning, retrospective, sprint demo and preparation time, daily stand up. 
Time for over head : The ones mandated by organization like all hands,  trainings,  meetings, any special projects from organization perspective,  recruitment, email checking  etc.  Scrum master will have to identify how much of over head is there and negotiate with stake holders to minimise them or to better organise them so as to eliminate disturbance to team's commitment. 

Time for unplanned : These activities are the ones that cannot be planned like impact of impediments,  unplanned but unavoidable meetings etc.  As the name suggests is not planable and hence reserve a effort bucket in alignment with your management and team which will not be planned. 

Net planable time : This is the capacity left over after taking away the above mentioned ones. Teams should only plan for this and provide commitment.