In a real-time system, new samples from the microphone are continually added, and the oldest samples are discarded. The sample rate would be something like 8,000 samples per second, this is reasonably good for capturing voice audio.
The buffer that stores the samples needs to be large enough to store the longest phrase you are trying to detect.
At a slower rate (perhaps 10 times per second), the trigger detection process is run on the entire buffer.