We're not sure exactly when this will happen, but the Model S is already nearing 400 miles.
According to current EPA estimates, the Tesla Model S Long Range offers 373 miles of range. The Model X Long Range comes in at 328 miles. However, during this week's Tesla Q4 2019 Earnings conference call, CEO Elon Musk admitted that Tesla hasn't actually updated the EPA's figures.
This means that the Model S and X are already likely to have more range on tap than the EPA's site suggests. As the conversation continued, Musk looked to another Tesla executive and asked if the Model S' range is now already in the 380s.
Musk went on to assert that a 400-mile range estimation for the Model S is in the cards. He shared, via Green Car Reports:
“We’re rapidly approaching a 400-mile range for Model S.”
A followup question inquired about 2170 cells, which are used in the Model 3 and rumored to be coming to the Model S and X eventually, though that's not actually the case. In fact, Musk said the cars will not get the 2170 cells. Instead, Tesla has continued using 18650 cells in its flagship vehicles.
Musk remarked that Tesla must be using energy efficiently. This is because 400 miles of range is a huge achievement from a battery pack that's the same size other automakers are using to get about half the EPA-rated range. Musk concluded about the Model S:
“We’re pretty happy with the energy content of the cell, and the improvements in the efficiency of the vehicle."