The bigger you go the better.
In real life situations I have seen they will only keep up a fraction of what you expect them to. Just witnessed a power failure at Sonisphere where we lost main generators and swap took about 10-15 minutes to go over to spares, 1 SC48, 2 Profile systems and some lights desks @ FOH all on independent UPS's and I did not see a single desk that was able to hold these minutes.
You are supposed to swap out the batteries like once a year but let's face it, who does? Also you are supposed to keep them charged but often racks stay in the warehouse for quite some time and then when they roll out and are powered for like half an hour and something happens to mains they won't have enough juice.
So the bigger you go, the better
