I don't have any direct experience, but if I had to give it a shot based on what I know about otehr CO2 setups, I would imagine that a 24 oz tank would give 3 months, possibly 4, at the rate that you are running it.
Honestly, at anything greater than a 29gal, my personal preferance is to use a 5# cylinder or larger. They tend to cost less to fill overall, and don't need to be filled as much. The initial investment is higher, but I think it is worth it.
However, if I had the choice again, I probably would go with a paintball setup on my 10gal. The 5# behemoth sitting in my stand takes up more room than I like. There is also the fact that it is cheaper overall, which is appealing as well.
Just wanted to get your thoughts as the last 2 paragraphs seem to contradict each other concerning fill costs.
Wouldn't the 5# or more setup cost less over time to fill? Also, above you recommended 2 20 oz. cylinders for less cost. Seems counterintuitive. Not saying your wrong, just trying to understand as I'm looking to do some setups as well.
Nice to see a fellow local. Let me know if you'd ever like to swap plants or stock sometime. I should be set-up in a couple months after I move.