how far out does the earth's magnetic field go into space?

Earth's magnetic field is dynamic and tear drop shaped. There is no one correct answer to your question.

http://www.flickr.com/photos/equinoxgraphics/5688252538/
 
Magnetic fields in general do not have a well defined edge. They just get weaker and weaker as you move away. So in a sense it goes on forever.
 
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