Without rehearsal, a short-term memory can vanish in under 20 seconds

In 1959, Lloyd and Margaret Peterson had people hear three letters, then count backward from a random number to block rehearsal. Recall collapsed fast — from nearly perfect at 3 seconds to around 10% by 18 seconds. Short-term memory fades automatically with time, not only when new information bumps it out.

— Lloyd R. Peterson and Margaret J. Peterson, Short-Term Retention of Individual Verbal Items — Journal of Experimental Psychology, 1959