I took some data a few months ago on converting sodium bicarbonate to sodium carbonate with heat:
At 0 hours, net mass 100 grams (0% converted)
At 1 hour, net mass 80 grams (54% converted)
At 2 hours, net mass 67 grams (90% converted)
At 3 hours, net mass 63.4 grams (99% converted)
This was baked at about 300F on an open electric hotplate in a shallow aluminum foil dish, sodium bicarbonate powder layer 1-2cm deep, sample surface temperature 105-110C, loosely covered to keep heat in but let CO2 and H2O out. The powder looks identical to me before and after conversion to sodium carbonate, just label your containers. Deeper layers of powder don't convert well, with the CO2 and H2O driven off the bottom layers cementing the top layers together. Eventually it all heats up and converts, but it takes a while. Higher temperatures would probably convert faster, although the CO2 and H2O from flame might slow down the conversion (which is why I used electric).
An open dish of sodium carbonate very slowly converts back to sodium bicarbonate, with a half-life in air of several weeks in my tests (dry cool winter indoor temperatures).
If you're dumping it in a melting dish as a flux, the flame for melting will probably rapidly do the work of converting sodium bicarbonate to sodium carbonate anyway!