How Do I Know When To Replace My Co2 Tank?

[Katch]

I have a dennerle regulator that I use with a 2kg fire extigisher. When I attach a new FE and fire it the top guage goes to around 60 psi. I then set the opperating pressure to just over 1 bar.

How do I know when to replace the CO2? I assume the pressure will drop in one of the 2 guages. Which one and how long after this drop before the FE dies?

 

[elisew]

I have the JBL pressurised co2 setup that I`ve been using for the last 3 weeks (ish), when the needle got to the red bar on the left hand gauge I had 3 days worth left (6 hrs a day) but had a spare cylinder to hook up straight away.