O2 comes in a variety of cylinder sizes. A 650 liter cylinder will last 65 minutes at a 10lpm flow rate. Less, of course, as you increase the flow. At 15lpm, an attack could take as long as 20 minutes to abort, giving you abort 4 aborts per E tank (the smaller cylinders). I have three E tanks I use for travel. One large H, or J, tank for home. The large tank holds about 1700 liters. I also use a demand flow valve on my large tank which regulates O2 flow to intake with nothing wasted as I breathe out. The higher the flow rate of O2, the more effective it becomes, and I can abort most attacks within 3-5 minutes. That's why we recommend a flow rate of 25lpm or more. It works faster and prolongs our tank's useful life. I would urge you to get a bigger tank for home, and investigate the demand flow valve system. Blessings. lance

Now I'm a little more with it (rough night) I called the supplier. The supplier delivers on a 24 hour basis and their engineer lives in my town. They're going to bring another two tomorrow and swap the one I have so I can make sure I always have 'one in-hand'. I like the size of the tank as it's not so big as to be difficult to move and besides I have to rely on my OH to move it due to my other health problems. I thought my GP was ordering an ambulatory supply but he didn't. If I needed to, I could take one if I go out in the car as I'm never far from it, again due to other health issues. I also have the option of a sumatriptan injection as these are effective for me in 10-15 minutes.

The supplier was very professional and helpful and said we can look at it again if it doesn't work out for me.

Thank you for your replies 

Good advice, Mike! I always tend to run low as the week approaches either a holiday or Friday. I've turned in tanks with some O2 left just because I didn't think it would last the duration. blessings. lance