This is definitely NOT a good idea. You will sooner or later, run into invalidsesskey issues, which with a session timeout that long.... you're creating a lot of work for yourself. Try setting the value to a school day, say
$CFG-->sessiontimeout = 28,800;
That is eight hours, and it forces a login every day. Also, don't forget the timeout is set to begin at the last action or keystroke of an activity. So you can realistically login at 8:00am, do a bit of work before noon, then do nothing until, say 6:00pm then at 1:00am do something else, and still not be logged out at 8:00am that morning. As Ken said, it can be done by hacking core, but there is the possibility of getting it wrong and creating more problems than you solve. Using the config.php file is safer.