This depends on the power consumption of the NAS (and all other devices connected to the UPS).
You need to look at the manuals and see the maximum power the device can consume.
Unless I'm mistaken, the power duration at full load means if the devices consume the maximum power supported by the UPS (here 540 W)
If we connect devices that consume half of this value (here 270 W) we have the duration at half load.
This is indicative and the time curve between 0 and maximum is not linear. (Some products give a more detailed graph).

Hello, I am using this UPS with a Synology DS918+ NAS and the battery power duration is about 20min.