In my experience, the FTTC batteries lasted 6 hours in my cabinet. I only know that because the road the cab was in suffered a power cut, but my road was unaffected.
The length of time the batteries last depends on the number of active ports on the DSLAM and the condition of the batteries.
8 hours is a good average but in reality it can range from 4-12 hours.
I'm sure someone will be along to report their cabinet died within an hour or 2 but that's definitely not the norm.
During a big power outage in Leicester a few years ago there was a user on kitz reporting losing sync for 10 minutes every 8 hours over 3-4 days.
It was the DSLAM running on batteries and those batteries being changed every 8 hours.
That's pretty decent imo, especially compared with backup power options on various other networks.
Edited by j0hn83 (Wed 02-Sep-20 16:30:14)