Negi Springfield --
Either set yourself a time limit and work until your reach it or work the spell as long as you please. There is no need to work it on and off, although some may be of a mind to only work this sort of trick during the waning or dark of the moon.
Papa Newt is right; there is no point in even attempting work that you do not believe in. Please read this page and see if it addresses your issues of doubt -- http://www.luckymojo.com/spells/real.html
It answers the following common questions from newbies:
HOW DO REAL MAGICK SPELLS ACTUALLY WORK?
DO MAGICK SPELLS ALWAYS WORK?
ARE MAGICK SPELLS GUARANTEED TO WORK?
IS MAGICK WORTH TRYING IF IT IS NOT GUARANTEED?
IS MAGICK REAL OR ARE THE RESULTS JUST COINCIDENCE?
IF MAGICK SPELLS ARE REAL, WHY DOESN'T EVERYONE USE THEM?
I'M A NEWBIE: HOW DIFFICULT ARE MAGIC SPELLS TO DO?
WHICH MAGICK SPELLS ARE THE STRONGEST OR MOST EFFECTIVE?
IF I CAST TOO MANY SPELLS WILL I GET "SPELL OVERLOAD"?
IF I CAST SPELLS INCORRECTLY WILL THEY "BACKFIRE" ON ME?
HOW LONG SHALL I WAIT FOR MAGICK SPELLS TO WORK?
DO YOU HAVE TO BE A CERTAIN RELIGION TO PRACTICE REAL MAGICK?
WHY ARE HOODOO SPELLS DONE WITH PSALMS OR IN JESUS' NAME?
WHERE CAN I LEARN MORE ABOUT REAL MAGICK?
If reading the above linked page does not clarify matters for you, i suggest you abandon the practice of magic, or at least those spells that do not work for you.