How many hours does it take a little heater to warm up a whole house?

Tough to say without knowing more about the size of the home, outside temperatures, and the output of the heater. Unless it is just 1-2 rooms, chances are the heater may never warm up an entire house. The amount of heat created is probably not enough to get the entire home above a certain temperature. It will warm it up some marginal amount but may not have the capacity to get it to your desired temperature.