Where to find/fix/configure "You've been idle..." logoff?

[cls@petra.greens.org]

I normally connect to my Debian systems via telnet.
My 1.3(bo) system lets me remain logged into a shell all day.
My 2.0(hamm) system logs me off after about an hour of idle time.

Where is this automatic logoff feature configured?
Documented?
How do I turn it off or change the timeout value?

The feature seems to be independent of which shell I'm using.
At least bash and zsh behave the same.

I've looked in the manpages for inetd, login, and telnetd.

The message is 
  cls: You've been idle for 64 min.
You'll be logged off in 30 sec unless you hit a key
and then it closes the connection.

TIA
Cameron
cls@greens.org