Depends how the car is driven? My Focus had the originol when i bought it after 3yrs and was still going strong after 7yrs. My mum (bless her) being a stop start driver due to her job usually only gets about 2-3yrs out of all the batterys shes had.
Stop start (particually in winter due to electrical load) will drain and wear your battery out quicker than a healthy run to and from work everyday would.