If you are new to wax melts and wondering when to change the wax melt over. You may think that wax melts evaporate but this is a very common misconception. The wax itself does not evaporate. However, the fragrance will dissipate and will no longer emit a nice fragrance. Once the scent becomes weak or non-existent, you should remove the wax melt cube and add a fresh one. Read this article on how to use wax melts if you are new to wax melts.
Unlike candles wax melts do not get hot enough to evaporate as they do not come in to direct contact with a flame. The flamed wick in a candle does get hot enough and the wax nearest the flame reaches high temperatures that in turn vaporizes and evaporates the surrounding wax.
When using candles you will notice the wax reducing while the candle burns. This is because the wax itself is being used as fuel for the wick to burn to produce heat, light and fragrance. Whereas wax melts only use the fragrance oil to produce the scent as the wax gets hot enough to dissipate the fragrance oils around the room but not hot enough to evaporate the wax itself.