I got myself a free broken UPS about a year ago, it's a 1KVA model. Course it was the batteries that were shot, so I had to buy a new set -- probably the biggest cost of a new ups. Anyway since then, I've only had one 1-minute power outage and the UPS was able to carry through, saving a crash/reboot. Since it's 1KVA it could carry through much longer, estimating around 45min to 1h reserve time before it runs out of energy.
The stupid thing is, I rarely get power outages here since power lines are buried in my neighborhood.
These larger models tend to have voltage regulation included. So if line power drops somewhat, it will increase it a bit, and same with overvoltage - it will decrease it. If it gets way out of whack in either direction, it will resort to using batteries.