I was planning on using a 2 litre bottle of Nitrogen. Was talking to a welder online and he said a 2 litre bottle (with a flow rate of 15lpm) will last for 26 minutes.
So yeah, I'm going to assume 40litres is a whole lot more than safe ;)
BTW, I don't know how accurate my first point is so don't go out and buy a 2 litre because I said. I'm a little skeptical about that myself. However, Max Dog Brewing (where I bought my regulator) recommend using this size cylinder:
And here's the cylinder I plan on using:
The one I plan on using is actually a bigger cylinder than the one used by Max Dog. Now I don't know if this means it has more gas inside or not so take that with a grain of salt.