Depends somewhat on how large the fire was before the power outage, but typically (at least from what I've seen) a coal fire will come back to life if the power comes back in 10-15 minutes. 15-20 is hit or miss and 20 or more minutes will need a restart.
If your stove has a high temperature limit switch on it, it will trip after several minutes of no power. This, of course, will prevent the stove from operating when the power does come back.
If you can get the generator fired up inside of 10 minutes you should be fine without the UPS.