Spells: Time Until Results?

Spells and Philtres
(Photo credit: Wikipedia)

What is a good rule of thumb to time the results of a spell or working? For me, it seems to be all over the map. I had a candle healing literally start to work over night, a runescript talisman take a week or two, and then other spells take a month or more.

Is there a point where you just assume the ritual failed? Is there a way of predicting how long the “wait” for results will take? I know not to be impatient, but I’d just like to get a general idea of what to watch for.