After 1 hour of inactivity the user is automatically logged out. This behavior may vary between different users.
If no timeouts are configured in SSHD, then this is most likely related to the type of login shell used.
"tcsh" has a default "auto logout" after 60 minutes.
"ksh" has a configurable session timeout.
If the user's shell is tcsh, then check the value of the "autologout" shell variable:
# set |grep autologout
If this returns a positive value, this is the number of minutes of inactivity before a user is automatically logged out. You could configure this to a different value by modifying the users ~/.cshrc, or the global /etc/profile file.
Alternatively, tcsh will not auto logout an X session. It will determine this by looking for the "DISPLAY" environment variable. In the case of OpenSSH you can enable "X11Forwarding" in the sshd_config file, this will automatically set the DISPLAY variable, and disable auto logouts.
Other shells have similar behavior, for example ksh uses the "TMOUT" environment variable.