The magnifying glass seemingly possesses durability, as indicated by the 100% that is present in the bottom half of the object's icon. After igniting an item with the magnifying glass, I did not notice a decrease in said durability. To test this further, I ignited 10 flammable items. There was no decrease in durability following each attempt. I repeated the experiment in a different world under a different save file and obtained the same results.
Simply light a flammable object on fire with the magnifying glass. Note the lack of a decrease in durability after each use.