Yes the CO2 will go down fast. You do pay a price for only carrying CO2, I hav never wanted to take that price, chance with CO2. How far you are going and whether you have a cell phone, etc will determine your choice in how and what you carry.

Oh ya... CO2 is not nuclear and has no "half life". Half life assumes breakdown and that is not what normal CO2 does, it effuses through the tube with time...