Well, lets think about it a bit.
Depending on the beer, I can easily go through a cornie keg per week. Of course, this isn't all going through my liver. My wife enjoys beer as much as I do and I often have friends stopping by for a beer or two so it's hard to say for sure exactly how much per week/month. APA's go fast, saisons go fast, smoked porters go slowly. This week, I'll probably brew 25 gals of homebrew because we've been busy prepping our house for sale and we're short on our favourite styles.
I have a very flexible work schedule which allows me to brew most of the time but I would say I've brewed at least once per week for the last 18 months or so. As I said, I have some friends who I'm going to set up with CO2 systems so that I can provide them with beer too. This is in return for all the favours they've done for me over the years. Let's assume 2 cornies per month for each of them. What does that give us? Let's say 30 gals per month on average with some upside for parties etc. Part of my problem is a lack of forecasting for future brews. For example, in order to try to keep the beer clear, I don't touch the cornies once they are in the freezer. So I'm often surprised when they blow and have no replacement of a favourite ready. I do try to be more regular now but it's not working perfectly yet. Also, I find that I have hard time letting things lager an appropriate amount of time because I'm out of a style I want. So, I'd really like to go to a system where I have 3 or 4 cornies at a time and then when I'm down to the last one, I can brew again.
Not really answering your question but my intent is to put in a "permanent" system so I'm not carrying the pots and cooler in and out all the time. Just not sure how much to spend and how big to go.