I think it all depends on your gauge, size of tank and rate of out gassing. I run my cylinder until it is empty (I would put it through the mangle if I could, to get the last drop out) and then replace with the spare from my garage. The empty cylinder is then exchanged for a full one the next time I am by my supplier.
My CO2 gauge though showing two clocks is a cheap single stage pressure reducer. My CO2 bottle is a small industrial cylinder that contains about 15lbs of gas when full. The tank out gasses very quickly due in part to its 10 gallon dry sump from the days when it was 'low tech'. I try for a medium tech set up as I like large structural plants which unfortunately play the devil with water flow/circulation.
Only ever had upside down fish when I was playing around with the addition of CO2 via pressured 'carbonated water'. Too difficult to control, all fish recovered. (This was in the days before CO2 systems were common place, about forty years ago)