Once Tesla sells its 200,000th US car the credit will be cut by 50% to $3,750 after a period of up to two quarters. That puts the likely cut time at July 2018.
I will say I'm not sure what price segment Tesla's endgame for the Model 3 is. I assume somewhere in the 25-35K mark, but there's a lot of variance in that range and without seeing the Model 3's trims and features it will be hard to judge where it should fall compared to the competition.
Tesla's site has impressively little basic information about the Model 3. Heck, I don't even know if it has electric windows (obviously it does, but my point is that Tesla's site lacks a 101 level spec sheet right now). Kind of ballsy to ask for a $1K downpayment for a vehicle they aren't telling you a darn thing about.
It's not a downpayment, it's an instantly refundable deposit - it's basically a free* option to reserve a spot at the front of the queue.
* free apart from the opportunity cost of investing $1k, which in a savings account or a money market fund might be less than $20 over two years...