How to bake baking soda to produce sodium carbonate?
The temperature needs to be high enough, that's all. 450°F should work fine.  (2-5 hours is a little silly, since the reaction is pretty quick once the baking soda itself is up to temperature.)

If you want to check that everything's gone okay, you can weigh the powder before and after. If it's converted properly, its mass should decrease to 63% (EDIT: fixed miscalculation, Matthias' number is the correct one) of the original. (Possibly a little further if there was some moisture in it.)