I believe they are guaranteed to burn for 8 hours, however most people burn them for a little while, then turn off the heat because they are very concentrated scent. You can extend their lifetime by freezing them as well. Personally, I use mine until I want to change to another scent, pop my tart burner into the freezer for a bit (helps the tart slide right out of the burner) & then I tuck them into a drawer or closet & use them potpourri-sachet style. The tarts in this lot are straight from Yankee's website. They should be very strong smelling & great quality. Hope that helps!