Obviously it depends on how much I'm running and how often I'm running it. But I don't even have a basic idea at all. The CO2 tank was given to me, I believe its a 5 pound tank, could be a 10 I'm not 100% sure. I just don't know what to expect. Is this something that would last me a few months? A few weeks? years? Centuries? I assume a lot of you have experience with a 5 pound (or so) tank. Want to share your experiences with me to get somewhat of a ballpark here? As always, thank you fish friends.