Let's set up SLA durations for resolving issues for different SLA's. To do it you need to add a new custom field with SLA type in the Administration\Custom fields menu.:

Setting up conditions for SLA levels

While creating a field - add it to the issue view screen. Then you need to set duration times for every SLA:

Go to Edit Set Up the Field:

For every SLA you should specify:

Notice. The highest conditions has more priority.