Are you asking how long the tanks themselves last or how long a fill
The oxy tanks will last a very long time but need to be inspected
occasionally. The frequency depend on the law.
How long the fill last depends on you. No body can say without know
a specific usage value; like 2 cu ft/day, or something. Comparing the
volume of the tanks should be straight forward and the welding
supply you get your fill at should be able to advise you. Ask them if
you bought the smaller tank, could you trade it in on a bigger tank.
Use the small tank for a while and measure your usage.