/etc/default/sh

This file (which does not exist by default) contains a value for the following parameter affecting the use of the shell /sbin/sh:

TIMEOUT

An integer value that specifies the number of seconds that can elapse without user activity before a shell will exit. If this value is 0, undefined, or the file /etc/default/sh does not exist (the default), the shell will wait for user input until it is explicitly terminated.