Goodness, probably a couple or few years. Now, are you talking without reboots or crashes?
I also ran a C128D non-stop for a few years running a BBS, only taking it down for updates.
One thing I found out about UPSs a loooooong time ago, true sine wave output is the ticket (and more expensive.) 600VA UPS for $30? Stepped approximation, and it's hard on your components. My SX-64 would not generate the 60Hz clock on a stepped-approx inverter.
