If you play at a field with all day air you could get the tank filled after each game. There are some estimate charts on web pages like
Otter SC Customs. On their chart you should get about 480 shots. The only way to now for sure is to shoot and count but just figure 450-500 and you should be good. You could pickup a 20oz CO2 tank for around $20-$30 and get around 1100 shots.