You can manage SLA's using HelpDesk plugin. You can read about it at Manage SLA. In that case you need to calculate SLA durations based on real work time and weekends. So, HelpDesk plugin lets you set up your unique work time calendar of your company.

Creating work time calendar

SLA is based on work time calendar, so please create and set up work time calendar of your company or make a default calendar from 00:00 till 23:59. To do it, enter Administration/Add-ons/HelpDesk/SLA settings menu:

You need to create new calendar here and define work and non-work days and work time.

Starting with HelpDesk 1.9.1 you can find "Breaks" button on this dialog. Click on it to set pause(s) for each day. Due date is calculated taking breaks in count.

Selecting work time calendar in SLA field

After creating and setting up work time calendar you need to select it in SLA field configuration:

Now SLA durations in the new issues will be calculated based on work time calendar.