@Ndy, I've made a mistake in my previous answer: once an SLA policy is applied to a ticket another one with the same condition will not work, so in the case of consequent notifications after one and two hour, the only option is to use Scheduler tasks. I've edited my previous post to not confuse somebody else.
Here is an example on configuring the task to notify a contact about tickets that are being unassigned more than 15 minutes.
Go to Settings => Scheduler and create a task.
Give it some name and set a schedule of task execution. In this case it will be executed every hour starting at specified time.
Configure a condition, to which ticket you want to apply an action. The task will check whether the ticket status title is "New", the "AssignedTo" field is empty and the ticket was created more than 15 minutes ago. To check the last condition, use date and time functions.
Now() returns current date and time.
AddMinutes() gets date and time in the first argument (which is
Now()) and a specified number of minutes from the second argument (which is negative, so the current time will be decreased by 15 minutes).
Configure an action to perform. In this case, you need to use "Send email" one. Specify a recipient (it can be somebody from a "Contacts" list or an arbitrary E-mail).
Type in a subject and configure a message body template, using tokens and snippets: