I don't see any range given in the man page for the idle option to
pppd. Often 0 means "don't do anything" (i.e., "no timeout"). The
fact that pppd/options.c initializes idle_time_limit to 0 suggests that
is the case here. And since idle_time_limit, is an int, I assume that
the full range of positive integers is allowed. Have I got that all
right?