the ESPs have (some kind of) RTC.
The NTP polling interval is handled in the background.
When you just try to get current time, you get the time from the RTC.
Meaning, a time(&now); // read the current time does not cause a NTP call - it reads the current time from its internal RTC.
The interval of the NTP calls is fixed, but you can change it. The example sketch (from the ZIP) also includes how to change it, but frankly, I seldom change the default.
Furthermore, there is an example how to define a callback when the NTP call is initated.