Wax melts should be changed whenever you no longer smell them. If you're using a single cube that is generally eight to twelve burning hours. Additional cubes, ideally no more than three at once, will extend that time by a few more hours.
When you have used some of your cubes and the wax has lost its scent, simply add a few drops of your favourite scented oil or perfume to get a few more hours use. As a guide use 5 drops of fragrance for every 2 cubes. You won't get quite as many hours out of the refurbished wax but it'll keep you going for a day or two while you're waiting for your next order from Santorina Scents.