The actual test should be
Connect your PC the UPS and Power up your PC.
Run furmark to maxout your GPU
Run prime95 to maxout your CPU
With that your PC should draw max current from your PSU.
Pull the Plug of UPS to force the UPS the power-up your computer. Then you will know how long UPS can sustain your PC at max load.
For normal usage(no furmark or prime95)... You can do the same pull the plug and let the UPS sustain your PC. At normal use the PC should be able to run longer.
With that you can decide and consider the other's input pertaining to quality of UPS to get.