Ultimately, we can't tell you exactly how long you can leave butter out. We have no problem with leaving butter out to soften as long as it is well-covered. It usually doesn't take us more than a couple days to go through a stick anyway! We've never had rancid butter in that time.
For complete instructions, see my weekly column. Note: The vinyl melts in just a few minutes and is warm to the touch when you pull it from the 200-degree oven (don't use a hotter oven or the vinyl will start releasing toxins). You can form it by hand immediately, but use leather gloves if you're nervous. All beginners in Record Upcycling should try a bowl.
It is important that the chocolate not reach temperatures of 115 degrees F or higher as it can end up lumpy and gritty. Other chocolates, such as the white variety, melt at around 110 degrees F and should never exceed that temperature. It is important to cook or melt chocolate slowly in order to ensure that the liquid state of it remains consistent and does not turn out lumpy and unusable.