I've had spells work years and years later. It's like they hole up somewhere and wait for you to finish the work you need to do. These are what you call lifetime spells.
But, yeah, seriously, 15 minutes to a year or more. Why so long or so short? It depends on how the garbage is around you has to break down. How the oils, herbs, salts work for you. As I read a Rootdoctor say, not all spells will work the same for each person, and not all spells will work successfully, if it was a success in the past. How do you know? Intution, ah, if something looks good to you at that moment, and it's "can do". Also spells work in layers, and things break off and fall away.
So, about spell casting, this is another reason to keep things pure, even though it takes longer. Because if you have the one day it will work (on a time delay), you want to see roses in your yard, not horrible nasty weeds.