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.
I guess we can see this target around 2020 if the price will follow the hash rate as incremental
Let's take just hash power as variable... As we can see https://bitcoinwisdom.com/bitcoin/difficulty
Every 7 months (more or less) the total hash rate trends to a 100%.
Nov 18 2016 281,800,917,193 10.68% 2,017,209,539 GH/s
May 23 2017 595,921,917,085 6.42% 4,265,775,241 GH/s
Dec 2017 8.000.000 GH/s
Jun 2018 16.000.000 GH/s
Jan 2019 32.000.000 GH/s
Aug 2019 64.000.000 GH/s
Around Feb 2020 we can see 120.000.000 GH/s
