If I got something along the lines of this:  Eaton 5E Tower UPS, 850VA / 480W does anyone have any idea how long it would be able to run a ONT for?  It wont need to run anything else, just an ONT, the rest of the gear is run via the house with solar/battery.

I have looked at: PowerShield PSDCMIN1218 Mini UPS 12v/1.5a but its runtime is around 120-150 minutes. Powercuts at my girlfriends place are regular and for extended periods of time (hence the solar/battery backup for the house) - the last one lasted close on 3 days.  Unfortunately where the ONT is there is no existing solar/battery setup - its on the other side of the property in the garage with network cables run from there to the house.  

I looked at the APC website to try and work things out but didnt have an awful lot of luck.  

A rough estimate to times is all I am after thanks!