Interceptors patrolling

Can anyone explain to me how interceptors deal damage when patrolling? They say it only deals damage after every 15 minutes right? But the problem is planes can be very fast and they tend to leave the range before the 15 minutes end. Does the damage still hit even if the 15min mark ends? If not, would it be better to use attack instead? Beginner here, please correct me if I'm wrong. Thank you!!