Answer:
91.30 minutes
Explanation:
Assuming that when you start charging the phone it had 0% charge, what you should do is a rule of three. 
That is, yes in 21 minutes I charge 23%, how long will it take to charge 100%. 
charge time charge percentage.
 21 minutes 23% 
 x minutes 100% 
 
Then it would be: 
x = 100 * 21/23 = 91.30 
So you would have to wait 91.30 minutes to wait for the full charge.