UPS Backup Time Guesstimate

I'm considering a UPS to power a light load. It's a combo cordless
phone/answering machine powered by AC to DC converter block. Input: 120v,
60hz, 8.5w. Output: 9vdc, 500ma.

A low cost refurb UPS (APC BK280) has specs as follows:

Output power capacity 280 VA, 180 Watts
Replacement battery cartridge RBC2
Typical backup time at half load 18.2 minutes
Typical backup time at full load 6.7 minutes

On the assumption that the cordless phone's batteries are fully charged when
a long outage begins, I'm thinking that even less than the indicated power
would be drawn to keep the answering base alive until the phone's batteries
get weak, a which point more power would be drawn for charging. The phone
would not be used during an outage, so its NiCad batteries would discharge
just from being idle.

What's your guesstimate of backup time running on UPS described above???

Thanks,
Len
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