I'm looking at a 20lb for myself, which would be shared between a 75 and a 10, so I think we are in the same boat.
One thing I would love to know though, is if people are estimating the time before refill assuming that co2 is going to be on 24/7
I know with myself that I am going to have mine on a timer, that way I'm not wasting any co2 and that way my fish aren't being gassed out.
No. Relatively few people have pressurized CO2 without a solenoid, as it makes absolutely no sense to run CO2 24/7. So the estimates you'll see for any given size of cylinder will generally have one agreed-upon timeframe. The estimates lower than that timeframe are usually a mix of 24/7 CO2 and "I've had a small leak for quite awhile but haven't figured it out yet". Estimates over that timeframe are generally people who think they're injecting more CO2 than they really are.
But, again, when most of us talk about a regulator, we're actually talking about a reg, solenoid, and needle/metering valve.