If your tank is in a cold environment it'll read lower than it should, like 400psi or so in a fridge and 800psi outside in room temp, roughly.
I just recently used up my first 5lb tank and (with tank at room temp, so 800psi was 'full') once it started dipping into the red I was able to dispense another full 5 gallons of beer and keep two already carbed kegs at 12psi or so. I also purged one recently filled keg.
It ran out very shortly thereafter.
I ran it until it was empty purposefully though, partly because I had to get it recertified (hydro tested) which requires an empty tank, and partly because it was Christmas and New Years and nothing was open to refill!