re: How long does a cylinder of xyz size last?
Take cylinder size (in ft3) and divide by gas flow rate (in ft3/hr). That will give you the hours of gas flow that that size cylinder at that flow rate will 'last'.
ex: Most small MIG machines run the shielding gas flow rate at ~20 ft3/hr. So an 80 ft3 cylinder will last for ~4 hours of gas flow time (not exactly the same as 'welding' time, because of preflow and post flow, but close most of the time.) A tiny little 20 ft3 cylinder will only last 1 hour at that flow rate of 20 ft3/hr.
By me, Tractor Supply doesn't have the greatest price on the cylinders themselves, but seems competitive on the refill pricing.
So, buy the cylinder and just get it refilled there!
CyberWeld or IOC or other online welding supply places will sell you (last I checked a while ago) an 80 ft3 cylinder delivered to your door. IIRC, pricing was usually around $200 or so.
So I checked. Cyberweld will sell you an 80 ft3 inert gas cylinder for $202 empty. Add $43.75 filled with argon (for TIG on anything or MIG on aluminum) or argon-CO2 (I assume that would be the 'standard' C25 mix)
http://store.cyberweld.com/shielgascyl8.html
IOC has the empty 80 ft3 cylinder for $187 currently. Free shipping too. Add in their current 'save $15 on an order over $150' promo and you are down to $172 for the empty cylinder.
https://www.weldingsuppliesfromioc.com/80-cf-welding-cylinder-for-argon-nitrogen-helium-and-mixes
But a big YMMV, as local pricing on cylinders and refills varies all over the place.
I will suggest that you get the 'biggest' cylinder you can, as the price of the 'gas' is actually mostly the price to do all the 'handing' of the cylinder(s) and not really the price of the contents themselves (helium excluded, as helium is just expensive over the past few years).
80 ft3 cylinders aren't that hard to handle. Smaller ones are smaller and lighter, sure, but they run out faster and the price/ft3 of gas is way higher. If you can get a bigger cylinder and 'handle' it, get that. IMHO.
