The fact that information must be repeated - known as maintenance rehearsal- in order to remain in short-term memory is evidence that short-term storage is:

This is evidence that short-term storage is brief.  According to the psychologist George Miller, short-term memory has a capacity of seven items on an average, (plus or minus two). For instance, this means, if an individual is asked to memorize a list of 20 words, he or she is likely to member seven words from the list on average.