Tesla has a proven track record of batteries going over 300,000 miles with less than 10% degradation which still leaves them with a longer range than anything else on the road. Even though that is over 20 years for the average driver Tesla is still shooting for over a million miles and may already be there. Unfortunately, it is going to take several years to prove that.
From what I have heard other brands are not doing so well. My son's Leaf has lost about 30% with only 50,000 miles on it. The new Leafs have a totally different battery and will probably do better but not as well as a Tesla.