Hard to predict how long CO2 will last in each tank as it is combo of lots of variables. Not only how much we are adding but how much we are losing! I have had a 20 gallon long and 75 gallon side by side and using the same amount of gas in both!
My first thought for CO2 tanks, is to go used as they are far better value in my area and my methods of use. That leaves me looking to the local Craigslist or local shopping type methods. Restaurant closeout dealers may also have tanks as they are used for soda and beer. Local music scene? Lots of bands use CO2 for the "fog" and bands break up all the time, selling off the tanks.
I would put lots of effort into finding a larger tank before going through the constant hassle of breaking down to refill and replace small cartridges. There is a lot of potential for damage as well as lost gas through leaks and the less often it is done, the better.
Regs and attached parts may be lost due to damage more often than due to wearing out! Even the best of us may cross-thread things at times?