There are lots of time to reach 21 BTC. Bitcoin wont be gone tomorrow and it is here to stay for eternity as long as community have faith in it. So having the luxury of having BTC for a long time, I can say there are lots of time to reach that 21 BTC goal. Just work hard and focus on getting it and i believe everyone can as long as they do not stop.
Achieving 21 btc is difficult but not impossible, if we work hard and give our best efforts then one day we will be surely holding that amount in our wallet, constant efforts will help you in achieving that goal.
Nothing is impossible if you want to achieve that big amount of bitcoin
. But you cant take that easily or achieve that it takes a lot of time and efforts, keyboard press and internet connection. If you are eager to achieve that big amount then you could get it in a matter of time 
If we are aiming on a certain thing, we could possibly reach it but we need to work hard for it and we must put effort in able to attain it. You could get that amount in a matter of time but it would depend on how fast you gonna gain bitcoin or how fast you could pile your saved bitcoins.