A nifty little aluminum heater. (http://img.hisupplier.com/var/userImages/2008-11/11/hisunrays$011813759(s).jpg)
First of all, since the heater is using radiation, we will be looking at the equation for heat and radiation. To start off, here is the equation for the net heat flow in radiation. The second equation beneath it is changed in order to find time.
Now for our variables; we are assuming that the heater is made from aluminium. To obtain the surface area of the radiator, I assumed a 1mx0.3mx1m sized radiator.
The mass of the air in the room is calculated below. I am assuming a small room that is 6mx5mx3m, (3m tall in consideration of objects present in a room that would normally be 4m tall) resulting in the volume of air to be 120m cubed. Now, as a cubic meter of air is about 1.3 kg, I calculated further to get a mass of 156 kg of air.
Furthermore, taking our change in temperature (23-2=21) and the specific heat of air:
Plugging it in gives us a heat of 3292.38J! Lastly, now that we have Q, we can plug everything back in...
Thus, we see that it takes 162.6s, almost 3 minutes, to fully heat a room to a nice and comfy 23C. So, next time you turn up the heater, don't fiddle with it until it fully heats the room up- you want your room to be heated not too hot, not too cold, but just right!