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Sometime in 2018. They have delivered less than 100K of the 200K cars required in the USA, so it will likely not happen until the Model 3 is shipping in quantity. Even then, you have a quarter after the 200K car is sold to get the full credit.
Tesla recently crossed 122k US sales. The rate of the Model 3 ramp in the 2nd half of this year will have a big impact on how quickly they get to 200k from here. Looking at the trend, my current estimate is Q2 2018, which means the full $7500 incentive would last until the end of Sept'18.
If Model 3 ramp goes faster than this estimate, then the date will pull in 3 or 6 months, but people will get their Model 3s sooner.