I think it depends a lot on the size of your home how long they last. I can smell mine 3-4 days in my small house. Using the Scentsey brand melts. Off Brands a day or two. I always use the ones that smell Like I have been baking all day!
It all depends. If you never shut it off and leave it on 24/7 you will go through them faster (obviously lol) otherwise they last forever and with scentsy you can pour the remainder of the wax back in the container if you wanna switch out scents. So you don't end up wasting any.