There is really no way to predict how long it will take for any particular spell to work. Spells work in their own time and pace. A spell that happens almost immediately one time might take a year or more to work another. Remember that the spell energy goes out into the Universe to change something into what you desire. That might take quite some time to work.
Also, no spell is guaranteed to work 100% of the time. Sometimes the magic just fizzles and nothing happens.
That being said, if a spell has no results after a full month from the time you cast it, you might want to consider casting it again just in case.