To find the average speed of an object, we find the change in the distance divided by the time over which that change occurs. So, the average speed is the overall speed during the entire time interval.
On the other hand, instantaneous speed is the speed at a particular moment in time.
To measure the instantaneous speed. we select a time interval that is short enough so that the speed does not change appreciably within the period of the measurement.
Hence, we need a very short time interval for the measurement of instantaneous speed.
Therefore, the answer is "short"