You start preparing as soon as you know you have something coming up.  You should work on it as often as you are comfortable with - to the point where you know, no matter what happens, you can do EXACTLY what YOU want to do with it. IE: nothing outside of you (rambunctious audience member, monitor screen going out, whaddeveah) is going to throw your performance.  If anything ever "feels wrong" vocally, take a step back and leave it alone for a bit.  I don't recommend working more than half an hour at a time.
You shouldn't be able to over practice to the point where it won't sound as good.  There are a lot of factors that affect a performance, and no matter what, it is NEVER the same twice.  Hopefully, the adrenaline rush you get when you perform live in front of people will help kick your performance up a level.
					
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"I hold the key to an open door....will I ever be free...?"