Telnet inactivity timeout

Here's an other quick question, unrelated to my existing issues:

How can I increase the inactivity timeout of a remote telnet session?

I've been using many remote telnet sessions, which I leave open while doing
other things, and they timeout too quickly, closing the connection before I
can go back to them to use them again. Sometimes I had some program running
that couldn't complete its job before the timeout and the jobs got
interrupted before it was finished, even when I ran them in the background,
hoping that when the telnet session would timeout, the background process
would be allowed to continue until done. But it kicks everything out when
timing out, so I really would like to increase this timeout.
I couldn't find any config info about that anywhere. There's no mention of
this in any man pages and googling it didn't bring up anything..

Is it built-in and not changeable?

(I stopped using ssh because of the constant attacks that causes the
disabling of the logins)

-- 
Didier Godefroy
mailto:[email protected]