If you are measuring a room, clap your hands once and count the seconds of decay (until the sound has died out). In a small room the decay time may be as low as 0.5 seconds or even less.
If you are measuring an electrical system there is usually a high-pass filter to remove the DC-component. The cutoff frequency of this high-pass filter usually determines the length of the impulse response.
The decay time for the high-pass-filter is given by:
Decay time = 1 / cutoff frequency
For example, below we show a figure of a loop-back electrical measurement of a sound card. We see that the high-pass cutoff frequency (-3 dB) is 10 Hz.
The decay time for this example is 1/10 = 0.1 seconds. So to perform an accurate measurement, the length of the impulse response should be at least 0.1 seconds.