a 4 foot tall bottle lasts us 6 months at work, sometimes doesn't get used for days but probably enough for you for 6 years. We buy from Air Liquide, I have a bottle that I bought for myself from them that's about 2.5 ft. tall that I just exchange when its empty, no refill and I don't have to re-certify the bottle after ten years, it just gets taken care of and no 70 dollar a year lease. I think the other suppliers do the same. Sorry I don't remember how much it costs to exchange.
