The actual duration takes Service Level into account when calculated. Go to Admin Console/Data/Codes/Service Level Agreements/Service Level. If i recall correctly the default service level is Bronze 8x5. Create a service level that reflects your support and make it as default.
The Service Level field is read only because Service Desk extrapolates it from the SLA of the service inserted on the SD.
Service has SLA and SLA has Service Level. When you insert Service on the service call form, service call tries to find if the service has an SLA and if it has, what is the service level. If it founds something it automatically insert to Service Call. If not it uses the default service level.
In order to define SLA you need license for Service Level Management.