One BTC is mined every ten minutes, and the frequency halves every 4 years so that the last BTC will be mined around 2140 after another 32 halving events. By 2136, today's daily quota of 144 BTC will be reduced to just 0.00000168 BTC.
https://bitcoin.stackexchange.com/qu...-mined-per-day
Ok, so that's a long way off, but to get there the frequency still halves every 4 years. McAfee (a mathematician) worked out that the cost of mining the last BTC would be from several million to several hundred million $$, and for the million or so before that the cost would rise from several hundred thousand $$ to several millions. Quite simply, the value of BTC must by those times justify the risk since mining comes with no guarantee of success. So in order for the last couple of million (only 5 million to go as of 2018) to be mined, the BTC price would need to be astronomical, otherwise they will never be mined.