When leaving my Laptop (VAIO CR120e) on without power, i only get 2 hours and 30min. why?

i wanted to see how long my battery would last if i left my laptop on without power. only MSN Messenger in the task bar but i wasn't using it. i set my battery plan to " Vaio Optimized " and i only get 2 hours and 30min. can someone exsplain to me if this is an issue or normal?