How long will a caterpillar stay in his cocoon?

Question

my 2 year old son found a caterpillar about a week and a half ago, he would not let me put it on a tree to let it be he had to keep it. I honestly thought it would die because it was not in his natural habitat. well i was wrong. i gave it fresh leaves every day and cleaned out the container we are keeping him in and he stayed alive. he started making his cocoon last night just with some silky web that was hard as a rock, then this morning while getting my oldest ready for school, i noticed he had made the entire cocoon. i was just wondering how long this whole caterpillar to butterfly change will take as i want to make sure i am here so i can let it free when it starts to hatch or whatever it does. i do not want it to die. my oldest is so excited about it and would mever forgive me if it died. anyways thanks to all who answer, Holly

Answer 1

Q9: How can I raise a caterpillar?
Q15: What do butterflies eat?
Q26: Can butterflies be damaged by handling? Can you damage a cocoon by handling it?
http://www.animalden.net/ai/bug/butterfly/butterfly-faq.htm
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HOW long does it take to make itself a butterfly?

http://www.duneland.com/garden/messages/69.html

There are many different cycles for butterflies and moths. Some hatch in the same season but some are meant as a winter protection for the insect to get through winter. Some even have the ability to stay in a certain stage if conditions are adverse for hatching or emerging. I have always kept mine outside in a shaded area and they seem to do fine although this was mostly through the summer time. Inside they may need some humidity, which could be a small bowl with water in it, kept in the container where the cocoon is. Don't forget that when it emerges it may need to be fed.
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I guess the answer is it depends on what kind of butterfly it will be.