When your high pressure gauge starts to show a decrease in gas pressure, you are reaching the end of your cylinder.
I use a dual stage regulator, so I wait until the cylinder is completely empty.
Some people refill once they start noticing a decrease in pressure (the pressure drops to (say) 500-600 PSI).
How long does it take to completely empty when a decrease in pressure is noticed? I think I been on 700 psi for two months now with no further decrease.