It really depends on what metric you are trying to represent.
If it's customer oriented I would say it's more of the time it takes for you to resolve (incident) or deliver (request) something.
Depending on how you work with your customers it might start from when the item is created, or maybe from when it was assigned, or even, for requests, after the requester's manager approves it.
If you want to know how much time a particular record is assigned to a team or person, you might want to look into Metrics for field duration.
Finally, you need to figure out if you want this time to be in hours or business hours, as then you will need to calculate the values on a schedule.
