What reserve time will I actually get at my site?

Not the datasheet figure. Four adjustments have to be applied to a nameplate capacity before it means anything at a real site:

• Your actual measured load, not the estimate
• Your cabinet temperature, not 25 °C
• End-of-life capacity, conventionally 80% of rated
• The end voltage at the equipment terminals after distribution drop, which must stay above the −40.5 V floor That last constraint is why telecom reserve tables are quoted to 1.75 V or 1.80 V per cell rather than to a flat battery. Ask for minutes at your load, at your temperature, at end of life, not amp-hours.