To pretty much completely disable SSH timeouts, simply adjust the following directives in /etc/ssh/sshd_config:
TCPKeepAlive yes ClientAliveInterval 30 ClientAliveCountMax 99999
EDIT: Once that’s changed, be sure to restart your ssh daemon.
SECURITY WARNING: If you remove users from your system, but they’re still connected via ssh, their connection may remain open indefinitely. Be sure to check all active ssh sessions after adjusting a user’s access.