You should read through the 'dry fasting' forum on fastingconnection.com and then read the 'questions for the fasting doc' section also and decide what you want to do. It's a lot to read, but it's better to spend the time than be sorry later.
There is no such thing as 'true thirst' afaik, but there is dehydration, which can be dangerous. From what I have read (!) up to three days are considered safe when staying out of the sun and avoiding hard physical activity. Everyone says that you should build up slowly.
It's important that you are prepared and have the right mindset. If you panic, fasting is bad for you, because you achieve the opposite physical state, imo.