If, like me, you get given figures like networks must be up for 95% of working days, and then told to report on that, you may find this post helpful. Recently, I spent some time trying to work out how to account for this in Service Desk Express and here is what I reckon:
Ok, so lets say that for a given service the percentage uptime required is 95.00%. That means that the percentage downtime allowed is 5.00%. We will also assume that the number of hours per week in your work schedule is 37.
Consequently, the number of minutes downtime allowed per week = (number of hours per week in work schedule * 60) * percentage downtime allowed = 111 minutes.
Now at this point you have to take a bit of a guess as you need to know roughly how many incidents you are likely to have per week for this service. For the sake of arguement we will assume 5.
Thus your downtime in minutes allowed per incident and consequently your duration for urgency = Downtime (minutes) allowed per week / Number of expected incidents per week = 22.2
This may have a few limitations but it allows you to bridge the gap between the people writing the SLAs and those implementing the Service Desk Tool!