The current length of a logged in account holder's session is 2 hours.
For your own security, TAB implements a time-out function after you have logged into your account. That is, if you remain logged on but do not place any bets or load any pages for 2 hours, you will have to login again. This is in case you have forgotten to logoff or left your browser open. This way no one will be able to use your Account unless they know your logon details.