Trenac is right - it depends how fast you use the gas. If you have a biowheel filter, with or without the wheel, or any similar design filter you will be using a lot of CO2 to get the pH to a good level.
But there is another important factor - who is filling your bottle. From my experience welding equipment dealers fill the bottles to full capacity.
I don't know how exactly they fill it or how they measure if the bottle is full but some places don't fill it to full capacity. Reading the high pressure gauge doesn't really help - to me that gauge doesn't tell much beyond "full" and "almost empty".
Your best bet is to stick to a place that you find through trial and error and you know it does a good job filling your bottle.
Actually you will be lucky if you can find anyone who will be willing to fill it. Most welding suppliers only do exchanges, and are limited in the size tanks they carry. A typical welder uses a 20 pound or larger. Ten pound maybe, anything smaller than that unlikely. All the welding suppliers I ever contacted in California and Oregon refuse to fill your tank. According to them it is too time consuming and expensive to fire up their system just to fill one tank. When they fire up their system they fill dozens of tanks, and then swap these to their customers empty tanks.
Why would you want a tank so small? I dont quite agree with Niko. What is the most amount of C02 you might pump in? 2 bubbles a second? What is the least amount 1 bubble every two seconds? I don't think you would see that much of a time difference between the two.
The difference between 1 bubble every 2 seconds and 2 bubbles per second is 3 fold. Thinks about it this way, 30 bubbles a minute or 120 per minute. Yo might want to try paint ball stores, they refill small ones all the time, or fire extinguisher places.