How much runtime will an offline UPS provide?

This is dependent upon the size in Ampere-hours (Ah) of the battery set installed in the uninterruptible power supply. Most standby UPS have a small battery and their inverter sections are only designed to provide up to 5minutes runtime at their full rated load. Longer runtimes are made possible by over-sizing the UPS device i.e. using a 1kVA in place of a 500VA which can double the runtime supply available from the UPS battery.