"At least by sometime in 2018. The Model 3 should be coming out of the plant at a rate of 400,000/year, 33,333/month. Assume 50% are exported to Europe and Asia. That's roughly 16,000 per month which would get them close to the top 20."
The 20,000 Model 3s a month Musk estimates for December is more than enough to get into the top U.S. 25. Remember, his cautious rollout involves deliveries limited to California at first, then move east through the U.S. Foreign countries (except maybe Canada) don't expect to see any Model 3s until well into 2018. Musk is also keenly aware that U.S. buyers are looking anxiously at the 200,000 U.S. sales limit which can get the full $7,500 tax credit. He may allow a few foreign sales as needed to get that magic vehicle to happen at the beginning of a quarter, in order to maximize the time the credit will be available, but I suspect that's about all.