"standard" usage will reach about 2/4 years before the degrade in mileage and performance warrants a replacement.
Not sure where you get your information from, but so far things are looking much better than that. The Tesla Roadster had a slightly older battery technology and they have about 80% of the battery capacity left after 100,000 miles. Still very usable. A couple of Model S have hit 70,000 miles and have ~95% of capacity left, so the Model S battery is better. The 8 year warranty is a fairly safe bet by Tesla.