They last about 4-6 weeks for most daily glasses wearers. It will vary depending on frequency of use, the environment, the frame material, etc.
To get the best lifetime for the product, when applying be sure to clean the frames really well with alcohol first and let it fully dry and also make sure hands are clean. It also helps to not to touch the adhesive side when applying the pad if possible, and to not stretch them at all when applying. They should be washed (with mild soap and warm water) regularly to remove any built up oils.