I usually use a votive blended wax for my tarts. I really dont think they would last for 40 hours, thats a long time. Mine usually keep there smell for a while. Tarts are pretty small, so you could probally get maybe 10 to 12 hours out of one. Also depends on how much FO you use. You burn them in a tart burner, with a unscented tealight. Good luck, they are the easiest thing you can make. I love making them.