A couple hours to over a year, if at all. It all depends on the skill level of the practitioner.
My personal belief is that spells are a result of the manifestation of an individuals energy work. The more someone is able to solidify their energy and connect it to the correct outlets the greater chance of a change occurring.
Patience and practice. I find also having the right mindset aids in spell working and energy work as well. Make sure you are focused and your mind is clear. But energy work is not instantaneous and its doesn't work like magic is portrayed in fairy tales. Takes a lot more work and results can greatly vary depending on each individual