Godslayer, just because you have a bigger tank doesn’t mean you use more CO2. I have a 10 lb bottle and it lasts 7 months on my 75 gallon tank. I have it at about 1 bubble per second.
Hence,
Now that I think about it, there are more variables than just the bottle size and volume of water that will effect the CO2 usage. But anyway, 3 weeks is definitely not normal.