I thought it was that the more intensely one worked (in other words, the more one worked past their anaerobic threshold), the higher one's VO2 Max became? Training increases VO2 Max, even in an 8-12 week period. A 15 - 20% increased in oxygen uptake is typical for the average inactive person who works in their anaerobic threshold (70-100% of the MHR) for 20-30mins + for 3-5 times a week. This is roughly an icrease of 35-42ml of oxygen/kg of body weight/min