Service Targets aren't really designed for that type of function (continuous notifications at X intervals). They are designed as 1-time (per ticket) objectives to be met at a certain status level.
What you could do is create a time-based rule that runs every 30 minutes to send notifications while criteria XYZ is met. This would mean the first notification would fire at an odd interval.. ie: the ticket could be 'responded' at 8:20am, and then the first 30-minute notification would go out 10 minutes later at 8:30am.
If that's ok, that's the easiest way. Else you have to get a little more complex with rules running more often, and that's generally less favourable.