If a furnace is loaded with fuel and something to smelt, then the player leaves the vicinity far enough to unload the chunk, the result is whatever progress was made in smelting the current item is lost. Since fuel time was used to get the item cook progress that far, that fuel time is also lost.

E.g.
1. Load a furnace with 8 cobblestone and 1 coal
2. Leave chunk so that it unloads
3. Return to find only 7 cobblestone cooked and the furnace finished with the coal