[drumminfish]

Well mine was on a shelf above the tank, so it had a hose about 4 feet long running to the tank, and I never had any apparent shortage of CO2 in a 20g...I'm not sure about the seals, as you can see I used rubber bungs and they were a tight fit, i never had any leakage that I saw...and the good thing about those bungs, if pressure ever built up too much, it would just pop the bung out instead of pumping the mixture into the tank.

But yeah, back to your question, unless the tubing leaks, I don't think you'll lose much if any CO2. The only loss I can think of would be lack of pressure pushing the CO2 up, but I don't really think you'll have a lack of pressure until the yeast are almost all dead, and at that point it's time to change the bottle anyways. (If CO2 is lighter than air, you wouldn't have that problem anyways, but I'm not a chemistry major)

<edit>Sorry I re-read that after I posted and realized that every time I meant to type "bung", I spelled it with an "o" instead of a "u" lol</edit>