A setting and internal counter that tracks time since last ad shown/closed and will reject calls to "ShowAd" if the timer is < Ad Cap setting.

eg:

Setting for Admob Interstitial : "Minimum time before allowing next ad" : 120 seconds

ShowAd() returns false if the internal timer since last ad closed is < 120 seconds

0 Seconds as default would just allow all calls to work.

Another term for this rather than a cap could be a "cooldown"