I worked for a company a few years ago were Sony Vaio's (similar to yours) was standard issue - even with a healthy battery the duration you can expect was really poor - ended up getting another one as a spare but as Richard says they are really expensive - I think at the time they were £150+
I am sure they did a few options - one was 1600 mah which gave you about 45mins (obviously depends what you are doing) and a 3200 mah (or was it 2700 mah) which was the one to go for.
Sony quoted up to 2 hours with a standard battery on their spec sheet - yeah right!!!!