Half of the measure. It would last the same amount of time as a quarter note would in 4 over 4 time signature. It just means less quarter notes in that measure.
Explanation: The bottom number, usually 4, indicates one measure. The number on top represents the beats per measure. For example, 3 over 4 would be a waltz (three beats), 4 over 4 would mean that there are four beats per measure.
