I find the disks to last at strong strength for 2-3 weeks. I use the vaportronic and start with one disk - after a couple of weeks I add a fresh disk, but keep the older one in there - I keep doing this until it fills up (5 disks) - at that point I know which disk is older and chuck it as I replace with a fresh one. Using this method, you can see how the essential oils evaporate slowly over time, and you always are assured of having good strength by adding a new one every 2-3 weeks as needed. Each of the 5 disks is a different shade of essential oil content, with the last one being completely spent.