The full $7,500 federal income tax credit is available until the manufacturer sells 200,000 vehicles domestically. Then, it begins to subside. All cars purchased in the calendar quarter when Tesla hits the 200,000 threshold will receive the full $7,500. (In other words, if they sell car #200,000 on July 5, then everyone who buys a car through September 30 still qualify for the full $7,500. Conversely, if they hit 200,000 on September 28, then only two more days of purchasers receive it.)
Starting in the first quarter after car #200,000 is sold, the credit reduces to $3,750. This figure stays in effect for two calendar quarters. Finally, for the third and fourth quarters after car #200,000 is sold, the credit reduces even further to $1,875. These reduced credits are limited only by time; Tesla could sell a million vehicles per month and all buyers would still receive these reduced credits.
To summarize: Tesla sells its 200,000th domestic vehicle July 5, 2018.
All cars purchased through September 30, 2018, are eligible for the $7,500 credit.
Cars purchased October 1, 2018, through March 31, 2019, are eligible for the reduced $3,750 credit.
Cars purchased April 1, 2019, through September 30, 2019, are eligible for the further reduced $1,875 credit.
Tesla is responsible for publicizing the eligibility for these credits. The Internal Revenue Service tries to keep up, but are notoriously way behind.