I would say what all u gonna do just cut the fondant as a shape of the guitar? I would say think about what you want to do with it and just time it after get done just put it in the ice box and it will harden but i wont do it more then 2 day advance. I hope this helps you.
I would not put it in the refrigerator to harden. 
You can make it as far in advance as you need to. If it's going to be a cut out of a guitar that is going to lay flat on the cake you'll want it to stay soft. You can lay it on a wax paper lined plate/cookie sheet/etc. and cover with plastic wrap and it will stay soft. I've made fondant cut outs days in advance and used this method.
If you're going to make a guitar that is more like a figure, you just mold your guitar and leave it to dry. I mix gumpaste with my fondant when making this type of decoration so that it hardens better. I've done these type of decorations up to 2 weeks in advance and as little as 2 days in advance.
Like I said, putting it in the fridge to "harden" is NOT a good idea. As soon as it starts to warm up condensation is going to occur. That can lead to your colors bleeding and sticky fondant.
Thanks for the suggestions, it is just going to be a cutout design, and I wanted to make sure I had a couple of days to play around with it so I could get it right. Wish me luck!
I'm sure it's going to turn out great! I checked out your photos, love your blue flower cake 
When you get the design you want just leave it at room tempature away from light so that your color doesn't fade and cover with plastic wrap. It will be just as it was when you first made it. Be sure to post a picture. I'm going to be making a guitar cake for my son soon and I'd like to see what you come up with.
LyndaJean your cake looks great! Great job on the guitar. I'm sure Nick loved it
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