Tools & Fabrication - What size argon/co2 bottle for garage welder?

I have a little 220v MIG welder that ive been using flux wire for. I did my mini tubs and alot of other fab work with it but im really tired of the spatter. I also am going to buy a Burkhart bumper support and need to weld to it so il need some 70ers wire and an argon mix. Most of my work will be with mild steel but when fabricating off of the bumper support its made of chromoly

What size bottles do you guys have and how long do they last? I dont want to be going and having it filled everytime I use it, I was thinking of trying to find a 60 cfm bottle.

Thanks!

Ive got a number 3 from tractor supply....lasted me just shy of 2 years. Did a lot of exhaust, some body work, SFC, welding random Sh--
Youll inevitable forget to shut the gas off for the night once or twice. Id just say get the biggest tank you have room for. At least for me getting it refilled sucks cause I have to drive an hour in the direction that is furthest from ANYTHING other than welding gas supply.

what he said^^^ but dont mig weld chromium molybdenum you should tig weld it

I have a 55 Cuft and its ok. Like everyone else said, buy as big as you can. I used to pay a dollar or 2 less per fill and that was a 20cu tank. Your burn rate depends on how much gas you need to run. When I weld steel I usually have it down to 5 cu ft per hour, but for the last week I have been welding stainless steel (Yes with a mig and the argon/co2 standard mix:)) and I had to bump it up to 20cu ft per hour.

Also I`m not so sure you can mig cromoly I thought that was tig only.

Ive got a 20cu.ft. and a 40cu.ft. The 40 cu is perfect for light duty/home garage mig stuff. Ive had mine filled 3 times in about 3 years. If your not trying to make money off of it just get the 40. Anything larger and they are pricey. Once you get on the gas you wont ever want to weld without it!!

Cool, I have a few 20s laying around but id like to find a 60 and not have to switch them out.

Ive welded chromoly before with a MIG, as long as you use an argon/co2 mix and a 70 ers wire it works just fine. I know there is a rule that states that chromoly needs to be TIG per NHRA rules but not sure if it only pertains to roll cages. My cage is TIG'd but this is for some front end work such as radiator support. If nothing else ill tack everything then take it to someone that can TIG it because I dont have the money or tiem to learn the skill.