Sometime in 2018.
21*2300=48300 in 6/2017.
With current difficulty level you could mine a bitcoin with an estimated $1000 cost per coin.
Whenever it costs $35000 to mine a coin then we could see a price around $48000.
With a wild calculation, when difficulty increases at least 30 times of the current level is when 21 bitcoins worth $1,000,000.
Not to be overly precise, but the goal of $48000 is only ~21 times the current price you quoted of $2300.