Block heaters have thermostats in them, so they heat up to a temp and then self shut off.
Every installed block heater has a different life span due to the way the element can heat the coolant / water, and many other factors.
Kubota engines are hard on heating elements because the tend not to get enough coolant / water around the element so heat dissipation and boiling occurs, when they boil off they tend to cause the element to spike in heat because the thermostat in the case of the block heater doesn't read the heat being too high.
A good setup can run 24/7 without issue but it's like anything, the more you run it the less time it will last.
And yes they can eat quite a bit of juice especially when outside temps reach -19 like it says you'll get.
My main question is are you going to use the tractor when it's that cold out?
I don't see any snow in your forecast, so a better bet is let it chill down and take a nap till this cold streak pops on past.