Results usually depend on the specific magick being done, just like with science or medicine results vary. For instance if I do a healing spell for a minor ache or pain, it can sometimes produce an almost immediate effect. A more serious injury however would likely require a lot longer for results to show.
Keep in mind that magick is not really supernatural but using natural (but sometimes misunderstood by the practitioners or clients) processes. Placebos and the power of suggestion are not to be ignored!
With regard to signs of a spell's success, it's pretty common for a magickal practitioner to use a form of divination (like the tarot, i-ching, etc.) to check before they do a spell to see if it is wise to proceed, and then again after the spell to see if it is likely to work.