There is no average. It depends on what substances you are injecting, the strength per ml, duration of the cycle, etc.
Notice that the 24,000 cc is over the course of 5 years. Still seems very, very high. 5 x 365 = 1825 days; 24,000 ml/1825 days = ~13 ml/day; 13 ml/day/2 buttocks = 6.5 ml/day in each buttock for 5 years straight. Doesn't add up. They must have meant 2400 cc over 5 years.