Start another timer in the code that makes the person appear. Start the timer with a unique timer number so you know when this timer ends. When the timer callback function is called with the unique timer number, call visible( viz.OFF ) on the avatar.
Code:
#With the code that starts the avatar
viz.starttimer( HIDE_AVATAR_TIMER, 5 )
#Timer callback
def myTimer( num ):
if num == HIDE_AVATAR_TIMER:
avatar.visible( viz.OFF )