How much time can you use a space heater in one day without it not costing a lot?

there are two tall ones that have wheels on bottom and two tiny box sized ones.

Our HVAC is not being worked on.

Back in 2022, for 1 month.. used one of the space heaters with the wheels on the bottom, for two hours at night.

I have tried and tried to figure this out and just can't figure out how to do this.

Very cold. ..

Not really using the space heaters, anymore except maybe 7 times since 2022

How much time can you use a space heater in one day without it not costing a lot?
Post Opinion