I worked out that the 'sparklet' bulbs hold 8g of CO2 which is about 4l or 7 pints of gas (at atmospheric pressure).
So, If you add 7 pints of gas, then you can draw off another 7 pints, right? It's not quite that simple though as when you add one to the barrel, some of it dissolves into the beer, and the amount dissolved depends on the pressure in the barrel, temperature, alcohol content, etc. In practice, you seem to be able to draw off 7 'nearly pints' immediately after.
It seems as though the secondary fermentation goes on for weeks and weeks as my 7 week mature Tom Caxton is still producing enough gas to deliver nearly a pint every day. It might well be different in the winter though as it is very temperature dependant.
I have had some barrels (notably a Woodforde's Wherry) that have not needed any extra gas and have still had loads of pressure even after the beer is below tap level. I've also had some that have needed 4 or 5 gas bulbs and I've had the last few pints flat when I've taken the lid off when nearly empty.
Generally though I only need one or two per barrel to help the beer catch up to my drinking speed!
I would be interested to know how much priming sugar is required to create 1 litre (2g) of gas. Maybe I could dig out my old chemistry school books and work it out, something to do with molar masses?
I use 2 budget barrels, but I only use the injector caps as I am not convinced that the basic plastic safety vent caps are that good at holding pressure over a long time. That said, the whole arrangement is a bit crude, it would be much better with a cap that didn't turn against the barrel that was tightened down with a separate locking ring. That way the seal would just be squashed rather than squashed and twisted at the same time. With my barrels, if you tighten the caps too tightly the sealing ring pops into the barrel.
Drinking: Tom Caxton Real Ale
Maturing: EDME Superbrew gold bitter
Kits in the cupboard waiting for barrel space: Woodforde's Wherry, EDME superbrew stout
Thinking about: Wine