I get about 100 days from 2Kg fire extinguisher in 180litres.
You have 6 times my tank volume that, so that's 100/6 -> 16 days.
Welding cylinders are 600gr odd, so a third my 2Kg, so down to 16/3 -> 5 days.
My CO2 injection has taken ages to get to that level

cool

of injection & flow, yours having a guess will initially be twice my consumption so 5/2 -> 2 1/2 days, that's 240gr CO2 per day.
So I estimate you will get only 2 1/2 days for £10 welding gas container, or more interestingly £1460 per year for in the end 85Kg of CO2. A bargain NOT.
May I suggest either a 5Kg fire extinguisher (£20 odd for old ones) or pub gas often £20 odd for 6.3Kg refill. A 6.3Kg will last you 6300/240 -> 26 days.