Hey,
the regulator measures the pressure of CO2 gas inside the cylinder. However, inside the cylinder CO2 is present in as gas and liquid forms. The regulator will only tell us the pressure of the gas (not the liquid). What happens is that as CO2 is consumed, the liquid converts to gas so that the overall pressure is maintained. Hence, our pressure gauge in the regulator is always fixed (usually around 50-60 bar). Yes there are tiny fluctuations according to room temperature. When the liquid CO2 finishes we are now left only with the gasous form. As we consume it the pressure decreases and this is the only time when we can the pressure decreasing in the pressure gauge. It will usually last 1-2 days depending on consumption rate, size of bottle, etc....
Hope this makes things a bit clearer.
Cheers,
GM