That's a tough question to answer accurately since there are too many factors involved. Knowing the history of the car is pretty important. Such as, was the battery ever completely discharged and re-charged? Was the car subjected to using a lot of the accessories. What type of weather conditions was the battery used in mostly. These things will shorten the battery life.
On my '94ES, used mostly in the NYC area, the original Lexus battery lasted about 4 years, and the second battery, a Sears Diehard Gold lasted about 6 years.
Personally, I would get a new battery. ;)