So my Smart Things motion sensors have a trigger called "Motion not detected for a duration". But if your Camera does not have that trigger, you could try something like this as long as it does trigger on motion.
The key is using Variables and making a counter out of Date & Time. You'll want to import all of these flows.
Every time motion is detected, your variable will be set to 30 (aka 30 minutes). Do import this flow first because it creates your variable:
Every 15 minutes, this flow will decrement your variable by 15:
Finally, this flow will turn off your light (or whatever you want) when the variable falls below zero:
Now, this won't be exactly every 30 minutes. But it'll work. Ideally you'll want that motion sensor trigger called "Motion not detected for a duration". You could also add more Date & Time triggers to the counter flow to get it closer towards exactly 30 minutes. But if you add to many you'll end up hitting the rate limit because you'll cause too many events per 24 hour period. Feel free to experiment with it.